CHAPTER VIII ELECTRICAL MEASURING INSTRUMENTSAn instrument designed to measure electromotive force (electrical pressure) is called avoltmeter. An instrument designed to measure volume of current is called anammeter.There are many forms of reliable meters for measuring current and voltage, but all are more or less expensive and out of the reach of an ordinary boy.Some meters are more carefully made than a watch, and are provided with fine hair-springs and jeweled bearings, but all depend upon the same principle for their action, namely, the mutual effects produced between a magnetic needle and a coil of insulated wire carrying a current of electricity.The little meters described in this chapter are simple and inexpensive but quite sensitive. Unlike a meter making use of a hair-spring, they will stand considerable rough handling, but of course should not be subjected to such treatment unnecessarily.Two types of meters are described. Both operate on exactly the same principle, but one is more elaborate than the other.A Simple Voltmeter and AmmeterA base-board five inches long, two and one-half inches wide and one-half inch thick is cut out of hard wood. In its center, cut a slot three-eighths of an inch wide and one and one-half inches long, with the slot running lengthwise the board. Along each side of the slot glue two small wooden blocks one and one-half inches long, one-quarter of an inch thick, and one-half of an inch high.Fig. 102.—*A*, Base, showing Slot. *B* and *C*, Sides and Top of the Bobbin. *D*, Base and Bobbin in Position.Fig. 102.—A, Base, showing Slot.BandC, Sides and Top of the Bobbin.D, Base and Bobbin in Position.When they are firmly in position, glue a strip of wood, two and one-half inches long, three-quarters of an inch wide and one-eighth inch thick to the top as shown by D in Figure 102.Using these as a support, wind a horizontal coil composed of 200 feet of No. 36 B. & S. gauge silk-covered wire.A needle is next made from a piece of watch-spring. It should be about one and one-quarter inches long, and one-eighth of an inch wide.Straighten it out by bending, and then heat the center in a small alcohol flame until the center is red-hot, taking care to keep the ends as cool as possible.The spring is mounted on a small steel shaft made by breaking up an ordinary sewing-needle. Make the piece one-half of an inch long. It must have very sharp points at both ends. The ends may be pointed by grinding.Fig. 103.—Arrangement of the Needle and Pointer.Fig. 103.—Arrangement of the Needle and Pointer.Bore a small hole just large enough to receive the needle through the center of the spring. Insert the needle in the hole and fasten it in the center by two small circular pieces of wood which fit tightly on the needle. A little glue or sealing-wax will serve to help make everything firm.The pointer is a piece of broom-straw, about three inches long. Bore a small hole in the top of one of the wooden clamps and insert the pointer in the hole, fastening it with a little glue. The pointer should be perfectly straight, and in a position at right angles to the spring.Bore a small hole in the bottom of one of the wooden clamps and glue a small wire nail in the hole. The purpose of the nail is to serve as a counterweight and keep the pointer in a vertical position.The spring should be magnetized by winding ten or twelve turns of magnet wire around one end and connecting it with a battery for a moment.Fig. 104.—A, Bearings. B, How the Needle is mounted.Fig. 104.—A, Bearings.B, How the Needle is mounted.The needle is mounted in two small pieces of thin sheet-brass, one inch long and one-half inch wide. Bend each strip at right angles in the middle, and at one-quarter of an inch from one end make a small dent by means of a pointed nail and a hammer.The strips are now slipped down in the center of the slot in the coil with the dents inside of the coil and exactly opposite one another. After the exact position is found, they may be fastened into position by two very small screws.The sharp-pointed sewing-needle, together with the magnetized spring, pointer, and counterweight, should slip down into the dents made in the strips and swing freely there. It may require a little filing and bending, but the work should be done patiently, because the proper working of the meter will depend upon having the needle swing freely and easily in its place.Fasten an upright board, four inches wide and one-quarter of an inch thick, to the base-board, back of the bobbin.Attach a piece of thick cardboard to the upright by means of small blocks, in such a position that the pointer swings very close to it but does not touch it.The meter is now complete, except for marking or calibrating the scale. The method of accomplishing this will be described farther on.Fig. 105.—The Completed Meter.Fig. 105.—The Completed Meter.If the meter is wound with No. 36 B. & S. gauge wire it is a voltmeter for measuring voltage. If it is wound with No. 16 B. & S. gauge wire it will constitute an ammeter for measuring amperes.A Portable Voltmeter and AmmeterThe bobbin upon which the wire is wound is illustrated in Figure 106. The wood is the Spanish cedar, of which cigar boxes are made. It should be one-eighth of an inch thick, and can be easily worked with a pocket-knife. In laying out the work, scratch the lines on the wood with the point of a darning-needle. Pencil lines are too thick to permit of accuracy in small work. The bobbin when finished must be perfectly true and square.The dimensions are best understood from the illustrations. In putting the bobbin together, do not use any nails. Use strong glue only.Two bobbins are required, one for the ammeter and one for the voltmeter. After completing the bobbins, sandpaper them and coat them with shellac.Fig. 106.—Details of the Bobbin.Fig. 106.—Details of the Bobbin.The bobbin for the ammeter is wound with No. 14 B. & S. double-cotton-covered magnet wire. The voltmeter requires No. 40 B. & S. silk-covered wire. In both cases the wire should be wound carefully in smooth, even layers. A small hole is bored in the flange through which to pass the end of the wire when starting the first layer. After finishing the winding, about six inches of wire should be left at both ends to make connection with the terminals. The whole winding is then given a coat of shellac. A strip of passe-partout tape, one-half of an inch wide wound over the wire around the bobbin will not only protect the wire from injury, but also give the bobbin a very neat appearance.The armature is a piece of soft steel one inch long, one-eighth of an inch thick and three-eighths wide. A one-eighth-inch hole is bored one-sixteenth of an inch above the center for the reception of the shaft. The center of gravity is thus thrown below the center of the mass of the armature, and the pointer will always return to zero if the instrument is level.The shaft is a piece of one-eighth-inch Bessemer steel rod, seven-sixteenths of an inch long. The ends are filed to a sharp knife-edge on the under side, as indicated in the figure.Fig. 107.—The Bobbin partly cut away so as to show the Bearing. Details of the Armature and Shaft.Fig. 107.—The Bobbin partly cut away so as to show the Bearing. Details of the Armature and Shaft.A one-sixteenth-inch hole is bored in the top of the armature to receive the lower end of the pointer, which is a piece of No. 16 aluminum wire, four and one-half inches long.After the holes have been bored, the armature is tempered so that it will retain its magnetism. It is heated to a bright red heat and dropped into a basin of strong salt water. The armature is then magnetized by rubbing one end against the pole of a strong magnet.The bearings are formed by two strips of thin sheet-brass, three-sixteenths of an inch wide, and one and one-quarter inches long, bent and glued to the sides of the bobbin.In the illustration, part of the bobbin is represented as cut away. The center of the bearing is bent out so that the end of the shaft will not come in contact with the sides of the bobbin. The top of the center is notched with a file to form a socket for the knife-edges of the shaft.Fig. 108.—Completed Voltmeter.Fig. 108.—Completed Voltmeter.The bobbin is glued to the center of a wooden base, seven inches long, four inches wide and three-quarters of an inch thick. The terminals of the coil lead down through two small holes in the base and thence to two large binding-posts. The wires are inlaid on the under side of the base, i.e., they pass from the holes to the binding-posts through two grooves. This precaution avoids the possibility of their becoming short-circuited or broken.The case is formed of two sides, a back and top of one-half-inch wood. It is six inches high, four inches wide, and two inches deep. A glass front slides in two shallow grooves cut in the wooden sides, one-eighth of an inch from the front.The case is held down to the base by four round-headed brass screws, which pass through the base into the sides. It is then easily removable in case it ever becomes necessary to repair or adjust the instrument.The meter and case, as illustrated in Figure 108, are intended for portable use and are so constructed that they will stand up. A small brass screw, long enough to pass all the way through the base, serves to level the instrument. If a little brass strip is placed in the slot in the screw-head and soldered so as to form what is known as a "winged screw," the adjustment may be made with the fingers and without the aid of a screw-driver.Where the instrument is intended for mounting upon a switch-board, it can be given a much better appearance by fitting with a smaller base, similar in size and shape to the top. The binding-posts are then mounted in the center of the sides.To calibrate the meters properly, they are compared with some standard. The scale is formed by a piece of white cardboard glued by two small blocks on the inside of the case. The various values are marked with a pen and ink. The glass front, therefore, cannot be put in place until they are located.The zero value on the meters will normally be in the center of the scale. When a current is passed through the bobbin, the armature tends to swing around at right angles to the turns of wire. But since the armature is pivoted above the center of the mass, when it swings, the center of gravity is displaced and exerts a pull in opposition to that of the bobbin, and the amount of swing indicated by the pointer will be greater as the current is stronger. The pointer will swing either to the right or the left, depending upon the direction in which the current passes through the bobbin. The pointer of the instrument illustrated in Figure 108 is at zero when at the extreme left of the scale. The pointer is bent to the left, so that the current will be registered when passing through the meter only in one direction, but the scale will have a greater range of values. It will also be necessary to cut a small groove in the base of the instrument in this case so that the armature will have plenty of room in which to swing.Fig. 109.—Circuits for Calibrating the Ammeter and Voltmeter.Fig. 109.—Circuits for Calibrating the Ammeter and Voltmeter.When calibrating the ammeter, it is placed in series with the standard meter, a set of strong batteries, and a rheostat. The rheostat is adjusted so that various current readings are obtained. The corresponding positions of the pointer on the meter being calibrated are then located for each value.The voltmeters must be placed in parallel, or shunt with each other, and in series with several battery cells. A switch is arranged so that the voltage of a varying number of cells may be passed through the meters. To secure fractional values of a volt, the rheostat is placed in shunt with the first cell of the battery. Then, by adjusting both the switch and the rheostat, any voltage within the maximum range of the battery may be secured.This means of regulating voltage is a common one, and of much use in wireless telegraph circuits, as will be explained later.When using the meters, it is always necessary that the ammeter shall be in series and the voltmeter in parallel or in shunt with the circuit.Galvanoscopes and GalvanometersIn the first part of Chapter V it was explained that several turns of wire surrounding a compass-needle would cause the needle to move and show a deflection if a current of electricity were sent through the coil.Such an instrument is called agalvanoscopeand may be used for detecting very feeble currents. A galvanoscope becomes agalvanometerby providing it with a scale so that the deflection may be measured.A galvanometer is really, in principle, an ammeter the scale of which has not been calibrated to read in amperes.Fig. 110.—Simple Compass Galvanoscope.Fig. 110.—Simple Compass Galvanoscope.A very simple galvanoscope may be made by winding fifty turns of No. 36 B. & S. gauge single-silk-covered wire around an ordinary pocket compass. The compass may be set in a block of wood, and the wood provided with binding-posts so that connections are easily made.Another variety of the same instrument is shown in Figure 111.Fig. 111.—Galvanoscope.Fig. 111.—Galvanoscope.Wind about twenty-five turns of No. 30 B. & S. gauge cotton-covered wire around the lower end of a glass tumbler. Leave about six inches of each end free for terminals, and then, after slipping the coil from the glass, tie the wire with thread in several places so that it will not unwind. Press two sides of the coil together so as to flatten it, and then attach it to a block of wood with some hot sealing-wax.Make a little wooden bridge as shown in Figure 111, and mount a compass-needle on it in the center. The compass-needle may be made out of a piece of spring-steel in the manner already described in Chapter I.Mount two binding-posts to the corners of the block, and connect the ends of the wire coil to them. Turn the block so that the needle points North and South and parallel to the coil of wire.If a battery is connected to the binding-posts, the needle will fly around to a position at right angles to that which it first occupied.An astatic galvanoscope is one having two needles with their poles in opposite directions. The word "astatic" means having no directive magnetic tendency. If the needles of an astatic pair are separated and pivoted separately, they will each point to North and South in the ordinary manner. But when connected together with the poles arranged in opposite directions they neutralize each other.An astatic needle requires but very little current in order to turn it either one way or the other, and for this reason an astatic galvanoscope is usually very sensitive.A simple instrument of this sort may be made by winding about fifty turns of No. 30-36 B. & S. gauge single-silk or cotton-insulated wire into a coil around a glass tumbler. After removing the coil from the glass, shape it into the form of an ellipse and fasten it to a small base-board.Separate the strands of wire at the top of the coil so that they are divided into two groups.Fig. 112.—Astatic Galvanoscope.Fig. 112.—Astatic Galvanoscope.Make a bridge or standard in the shape of an inverted U out of thin wooden strips and fasten it to the block.The needles are ordinary sewing-needles which have been magnetized and shoved through a small carrier-bar, made from a strip of cardboard, with their poles opposite one another, as shown in the illustration.Fig. 113.—Astatic Needles.Fig. 113.—Astatic Needles.They may be held in place in the cardboard strip by a small drop of sealing-wax.A small hole is punched in the top of the carrier, through which to pass the end of a thread. The upper end of the thread passes through a hole in the bridge and is tied to a small screw-eye in the center of the upper side of the bridge.The carrier-bar is passed through the space where the coil is split at the top. The lower needle should hang in the center of the coil. The upper needle should be above and outside the coil.The terminals of the coil are connected to two binding-posts mounted on the base-block.Owing to the fact that this galvanoscope is fitted with an astatic needle, the instrument does not have to be turned so that the coil may face North and South. The slightest current of electricity passing into the coil will instantly affect the needles.An astatic galvanometer for the detection of exceedingly weak currents and for use in connection with a "Wheatstone bridge" for measuring resistance, as described farther on, will form a valuable addition to the laboratory of the boy electrician.Make two small bobbins similar to those already described in connection with the volt and ammeter, but twice as long, as shown in Figure 114.Wind each of the bobbins in the same direction with No. 36 silk-covered or cotton-covered wire, leaving about six inches free at the ends for connection to the binding-posts.Fasten each of the bobbins to the base-board with glue. Do not nail or screw them in position, because the presence of nails or screws may impair the sensitiveness of the instrument. In mounting the bobbins, leave about one-sixteenth of an inch of space between the inside flanges, through which the needle may pass.Connect the coils wound on the bobbins so that the end of the outside layer of the first coil is connected to the inside layer of the other coil. This arrangement is so that the current will travel through the windings in the same continuous direction, exactly the same as though the bobbin were one continuous spool.Fig. 114.—Bobbin for Astatic Galvanometer.Fig. 114.—Bobbin for Astatic Galvanometer.Magnetize two small sewing-needles and mount them in a paper stirrup made from good, strong paper, as shown in Figure 114. Take care that the poles are reversed so that the north pole of one magnet will be on the same side of the stirrup as the south pole of the other. They may be fastened securely by a drop of shellac or melted sealing-wax.Cut out a cardboard disk and divide it into degrees as in Figure 115. Glue the disk to the top of the bobbins. A small slot should be cut in the disk so that it will pass the lower needle.A wooden post should be glued to the back of the base. To the top of this post is fastened an arm from which are suspended the magnetic needles.A fine fiber for suspending the needle may be secured by unraveling a piece of embroidery silk.Fig. 115.—Completed Astatic Galvanometer.Fig. 115.—Completed Astatic Galvanometer.The upper end of the fiber is tied to a small hook in the end of the arm. The wire hook may be twisted so that the needles may be brought to zero on the scale. Zero should lie on a line parallel to the two coils.The fiber used for suspending the needles should be as fine as possible. The finer the fiber is, the more sensitive will the instrument be.The lower needle should swing inside of the two coils, and the upper needle above the disk.How to Make a Wheatstone BridgeThe amateur experimenter will find many occasions when it is desirable to know the resistance of some of his electrical apparatus. Telephone receivers, telegraph relays, etc., are all graded according to their resistance in ohms. The measurement of resistance in any electrical instrument or circuit is usually accomplished by comparing its resistance with that of some known circuit, such as a coil of wire which has been previously tested.The simplest method of measuring resistance is by means of a device known as the Wheatstone bridge. This instrument is very simple but at the same time is remarkably sensitive if properly made. A Wheatstone bridge is shown in Figure 116.The base is a piece of well-seasoned hard wood, thirty inches long, six inches wide, and three-quarters of an inch thick.Secure a long strip of No. 18 B. & S. gauge sheet-copper, one inch wide, and cut it into three pieces, making two of the pieces three inches long, and the other piece twenty-three and one-half inches long.Mount the copper strips on the base, as shown, being very careful to make the distance between the inside edges of the end-pieces just twenty-five inches. The strips should be fastened to the base with small round-headed brass screws. Mount two binding-posts on each of the short strips in the positions shown in the illustration, and three on the long strip. These binding-posts should pass through the base and make firm contact with the strips.Fig. 116.—Wheatstone Bridge.Fig. 116.—Wheatstone Bridge.Then make a paper scale twenty-five inches long, and divide it into one hundred equal divisions one-quarter of an inch long. Mark every fifth division with a slightly longer line, and every tenth division with a double-length line.Start at one end and number every ten divisions, then start at the other end and number them back, so that the scale reads 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, from right to left at the top and 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, from left to right at the bottom.Solder a piece of No. 30 B. & S. gauge German-silver wire to one of the short copper strips opposite the end of the scale, and then stretch it tightly across the scale and solder it to the strip at the other end.Make a knife-contact by flattening a piece of heavy copper wire as shown in Figure 117. Solder a piece of flexible wire, such as "lamp cord," at the other end. It is well to fit the contact with a small wooden handle, made by boring out a piece of dowel.The instrument is now practically complete.Fig. 117.—Knife-Contact.Fig. 117.—Knife-Contact.In order to use the Wheatstone bridge, it is necessary to have a set of resistances of known value. The resistance of any unknown circuit or piece of apparatus is found by comparing it with one of the known coils. It is just like going to a store and buying a pound of sugar. The grocer weighs out the sugar by balancing it on the scales with an iron weight of known value, and taking it for granted that the weight is correct, we would say that we have one, five, or ten pounds of sugar, as the case may be.The Wheatstone bridge might be called a pair of "electrical scales" for weighing resistance by comparing an unknown coil with one which we know has a certain value.The next step is to make up some standard resistance coils. Secure some No. 32 B. & S. gauge single-cotton-covered wire from an electrical dealer and cut into the following lengths, laying it straight on the floor but using care not to pull or stretch it.1/2 ohm coil—3 feet 1/2 inch1 ohm coil—6 feet 1 1/4 inches2 ohm coil—12 feet 2 1/2 inches5 ohm coil—30 feet 6 1/4 inches10 ohm coil—61 feet20 ohm coil—122 feet30 ohm coil—183 feet50 ohm coil—305 feetThese lengths of wire are then wrapped on the spools in the following manner.Fig. 118.—Resistance-Coil.Fig. 118.—Resistance-Coil.Ashows how the Wire is doubled and wound on the Spool.Bis the completed Coil.This method of winding is known as the non-inductive method, because the windings do not generate a magnetic field, which might affect the galvanometer needle used in connection with the Wheatstone bridge as described later on.Each length of wire should be doubled exactly in the middle, then wrapped on the spools like a single wire, the two ends being left free for soldering to the terminals as shown in Figure 118, B.The spools may be the ordinary reels upon which cotton and sewing-silk are wrapped.The terminals of the spools are pieces of stout copper wire, No. 12 or No. 14 B. & S. gauge. Two pieces of wire about three inches long are driven into holes bored in the ends of each spool. A small drop of solder is used permanently to secure the ends of the coil to each of the heavy wire terminals.The spools are then dipped into a pan of molten paraffin and boiled until the air bubbles cease to rise.The spools should be marked 1, 2, 10, 20, 30, and 50, according to the amount of wire each one contains as indicated in the table above.How to Use a Wheatstone Bridge for Measuring ResistanceThe instrument is connected as in Figure 116.The unknown resistance or device to be measured is connected across the gap atB. One of the standard known coils is connected across the gap atA. A sensitive galvanometer or a telephone receiver and two cells of battery are also connected as shown.If a telephone receiver is used, place it to the ear. If a galvanometer is used instead, watch the needle carefully. Then move the sharp edge of the knife-contact over the scale along the German-silver "slide wire" until a point is reached when there is no deflection of the needle or no sound in the telephone receiver.If this point lies very far on one side or the other of the center division on the scale, substitute the next higher or lower known resistance spool until the point falls as near as possible to the center of the scale.When this point is found, note the reading on the scale carefully. Now comes the hardest part. Almost all my readers have no doubt progressed far enough in arithmetic to be able to carry on the following simple calculation in proportion which must be made in order to find out the resistance of the unknown coil.The unknown resistance, connected toB, bears the same ratio to the known coil, atA, that the number of divisions between the knife-contact and the right-hand end of the scale (lower row of figures) bears to the number of divisions between the knife-edge and the left-hand end of the scale (upper row of figures).We will suppose that a 5-ohm coil was used atAin a test, and the needle of the galvanometer stopped swinging when the knife-contact rested on the 60th division from the left-hand end, or on the 40th from the right. Then, in order to find the value of the unknown resistance atB, it is simply necessary to multiply the standard resistance atAby the number of left-hand divisions and divide the product by the number of right-hand divisions. The answer will be the resistance ofBin ohms.The calculation in this case would be as follows:5 X 40 = 200200/60 = 3.33 ohms3.33 ohms is the resistance ofB.This explanation may seem very long and complex, but if you will study it carefully you will find it to be very simple. When once you master it, you will be enabled to make many measurements of resistance which will add greatly to the interest and value of your experiments.BELLS, ALARMS, AND ANNUNCIATORS
CHAPTER VIII ELECTRICAL MEASURING INSTRUMENTSAn instrument designed to measure electromotive force (electrical pressure) is called avoltmeter. An instrument designed to measure volume of current is called anammeter.There are many forms of reliable meters for measuring current and voltage, but all are more or less expensive and out of the reach of an ordinary boy.Some meters are more carefully made than a watch, and are provided with fine hair-springs and jeweled bearings, but all depend upon the same principle for their action, namely, the mutual effects produced between a magnetic needle and a coil of insulated wire carrying a current of electricity.The little meters described in this chapter are simple and inexpensive but quite sensitive. Unlike a meter making use of a hair-spring, they will stand considerable rough handling, but of course should not be subjected to such treatment unnecessarily.Two types of meters are described. Both operate on exactly the same principle, but one is more elaborate than the other.A Simple Voltmeter and AmmeterA base-board five inches long, two and one-half inches wide and one-half inch thick is cut out of hard wood. In its center, cut a slot three-eighths of an inch wide and one and one-half inches long, with the slot running lengthwise the board. Along each side of the slot glue two small wooden blocks one and one-half inches long, one-quarter of an inch thick, and one-half of an inch high.Fig. 102.—*A*, Base, showing Slot. *B* and *C*, Sides and Top of the Bobbin. *D*, Base and Bobbin in Position.Fig. 102.—A, Base, showing Slot.BandC, Sides and Top of the Bobbin.D, Base and Bobbin in Position.When they are firmly in position, glue a strip of wood, two and one-half inches long, three-quarters of an inch wide and one-eighth inch thick to the top as shown by D in Figure 102.Using these as a support, wind a horizontal coil composed of 200 feet of No. 36 B. & S. gauge silk-covered wire.A needle is next made from a piece of watch-spring. It should be about one and one-quarter inches long, and one-eighth of an inch wide.Straighten it out by bending, and then heat the center in a small alcohol flame until the center is red-hot, taking care to keep the ends as cool as possible.The spring is mounted on a small steel shaft made by breaking up an ordinary sewing-needle. Make the piece one-half of an inch long. It must have very sharp points at both ends. The ends may be pointed by grinding.Fig. 103.—Arrangement of the Needle and Pointer.Fig. 103.—Arrangement of the Needle and Pointer.Bore a small hole just large enough to receive the needle through the center of the spring. Insert the needle in the hole and fasten it in the center by two small circular pieces of wood which fit tightly on the needle. A little glue or sealing-wax will serve to help make everything firm.The pointer is a piece of broom-straw, about three inches long. Bore a small hole in the top of one of the wooden clamps and insert the pointer in the hole, fastening it with a little glue. The pointer should be perfectly straight, and in a position at right angles to the spring.Bore a small hole in the bottom of one of the wooden clamps and glue a small wire nail in the hole. The purpose of the nail is to serve as a counterweight and keep the pointer in a vertical position.The spring should be magnetized by winding ten or twelve turns of magnet wire around one end and connecting it with a battery for a moment.Fig. 104.—A, Bearings. B, How the Needle is mounted.Fig. 104.—A, Bearings.B, How the Needle is mounted.The needle is mounted in two small pieces of thin sheet-brass, one inch long and one-half inch wide. Bend each strip at right angles in the middle, and at one-quarter of an inch from one end make a small dent by means of a pointed nail and a hammer.The strips are now slipped down in the center of the slot in the coil with the dents inside of the coil and exactly opposite one another. After the exact position is found, they may be fastened into position by two very small screws.The sharp-pointed sewing-needle, together with the magnetized spring, pointer, and counterweight, should slip down into the dents made in the strips and swing freely there. It may require a little filing and bending, but the work should be done patiently, because the proper working of the meter will depend upon having the needle swing freely and easily in its place.Fasten an upright board, four inches wide and one-quarter of an inch thick, to the base-board, back of the bobbin.Attach a piece of thick cardboard to the upright by means of small blocks, in such a position that the pointer swings very close to it but does not touch it.The meter is now complete, except for marking or calibrating the scale. The method of accomplishing this will be described farther on.Fig. 105.—The Completed Meter.Fig. 105.—The Completed Meter.If the meter is wound with No. 36 B. & S. gauge wire it is a voltmeter for measuring voltage. If it is wound with No. 16 B. & S. gauge wire it will constitute an ammeter for measuring amperes.A Portable Voltmeter and AmmeterThe bobbin upon which the wire is wound is illustrated in Figure 106. The wood is the Spanish cedar, of which cigar boxes are made. It should be one-eighth of an inch thick, and can be easily worked with a pocket-knife. In laying out the work, scratch the lines on the wood with the point of a darning-needle. Pencil lines are too thick to permit of accuracy in small work. The bobbin when finished must be perfectly true and square.The dimensions are best understood from the illustrations. In putting the bobbin together, do not use any nails. Use strong glue only.Two bobbins are required, one for the ammeter and one for the voltmeter. After completing the bobbins, sandpaper them and coat them with shellac.Fig. 106.—Details of the Bobbin.Fig. 106.—Details of the Bobbin.The bobbin for the ammeter is wound with No. 14 B. & S. double-cotton-covered magnet wire. The voltmeter requires No. 40 B. & S. silk-covered wire. In both cases the wire should be wound carefully in smooth, even layers. A small hole is bored in the flange through which to pass the end of the wire when starting the first layer. After finishing the winding, about six inches of wire should be left at both ends to make connection with the terminals. The whole winding is then given a coat of shellac. A strip of passe-partout tape, one-half of an inch wide wound over the wire around the bobbin will not only protect the wire from injury, but also give the bobbin a very neat appearance.The armature is a piece of soft steel one inch long, one-eighth of an inch thick and three-eighths wide. A one-eighth-inch hole is bored one-sixteenth of an inch above the center for the reception of the shaft. The center of gravity is thus thrown below the center of the mass of the armature, and the pointer will always return to zero if the instrument is level.The shaft is a piece of one-eighth-inch Bessemer steel rod, seven-sixteenths of an inch long. The ends are filed to a sharp knife-edge on the under side, as indicated in the figure.Fig. 107.—The Bobbin partly cut away so as to show the Bearing. Details of the Armature and Shaft.Fig. 107.—The Bobbin partly cut away so as to show the Bearing. Details of the Armature and Shaft.A one-sixteenth-inch hole is bored in the top of the armature to receive the lower end of the pointer, which is a piece of No. 16 aluminum wire, four and one-half inches long.After the holes have been bored, the armature is tempered so that it will retain its magnetism. It is heated to a bright red heat and dropped into a basin of strong salt water. The armature is then magnetized by rubbing one end against the pole of a strong magnet.The bearings are formed by two strips of thin sheet-brass, three-sixteenths of an inch wide, and one and one-quarter inches long, bent and glued to the sides of the bobbin.In the illustration, part of the bobbin is represented as cut away. The center of the bearing is bent out so that the end of the shaft will not come in contact with the sides of the bobbin. The top of the center is notched with a file to form a socket for the knife-edges of the shaft.Fig. 108.—Completed Voltmeter.Fig. 108.—Completed Voltmeter.The bobbin is glued to the center of a wooden base, seven inches long, four inches wide and three-quarters of an inch thick. The terminals of the coil lead down through two small holes in the base and thence to two large binding-posts. The wires are inlaid on the under side of the base, i.e., they pass from the holes to the binding-posts through two grooves. This precaution avoids the possibility of their becoming short-circuited or broken.The case is formed of two sides, a back and top of one-half-inch wood. It is six inches high, four inches wide, and two inches deep. A glass front slides in two shallow grooves cut in the wooden sides, one-eighth of an inch from the front.The case is held down to the base by four round-headed brass screws, which pass through the base into the sides. It is then easily removable in case it ever becomes necessary to repair or adjust the instrument.The meter and case, as illustrated in Figure 108, are intended for portable use and are so constructed that they will stand up. A small brass screw, long enough to pass all the way through the base, serves to level the instrument. If a little brass strip is placed in the slot in the screw-head and soldered so as to form what is known as a "winged screw," the adjustment may be made with the fingers and without the aid of a screw-driver.Where the instrument is intended for mounting upon a switch-board, it can be given a much better appearance by fitting with a smaller base, similar in size and shape to the top. The binding-posts are then mounted in the center of the sides.To calibrate the meters properly, they are compared with some standard. The scale is formed by a piece of white cardboard glued by two small blocks on the inside of the case. The various values are marked with a pen and ink. The glass front, therefore, cannot be put in place until they are located.The zero value on the meters will normally be in the center of the scale. When a current is passed through the bobbin, the armature tends to swing around at right angles to the turns of wire. But since the armature is pivoted above the center of the mass, when it swings, the center of gravity is displaced and exerts a pull in opposition to that of the bobbin, and the amount of swing indicated by the pointer will be greater as the current is stronger. The pointer will swing either to the right or the left, depending upon the direction in which the current passes through the bobbin. The pointer of the instrument illustrated in Figure 108 is at zero when at the extreme left of the scale. The pointer is bent to the left, so that the current will be registered when passing through the meter only in one direction, but the scale will have a greater range of values. It will also be necessary to cut a small groove in the base of the instrument in this case so that the armature will have plenty of room in which to swing.Fig. 109.—Circuits for Calibrating the Ammeter and Voltmeter.Fig. 109.—Circuits for Calibrating the Ammeter and Voltmeter.When calibrating the ammeter, it is placed in series with the standard meter, a set of strong batteries, and a rheostat. The rheostat is adjusted so that various current readings are obtained. The corresponding positions of the pointer on the meter being calibrated are then located for each value.The voltmeters must be placed in parallel, or shunt with each other, and in series with several battery cells. A switch is arranged so that the voltage of a varying number of cells may be passed through the meters. To secure fractional values of a volt, the rheostat is placed in shunt with the first cell of the battery. Then, by adjusting both the switch and the rheostat, any voltage within the maximum range of the battery may be secured.This means of regulating voltage is a common one, and of much use in wireless telegraph circuits, as will be explained later.When using the meters, it is always necessary that the ammeter shall be in series and the voltmeter in parallel or in shunt with the circuit.Galvanoscopes and GalvanometersIn the first part of Chapter V it was explained that several turns of wire surrounding a compass-needle would cause the needle to move and show a deflection if a current of electricity were sent through the coil.Such an instrument is called agalvanoscopeand may be used for detecting very feeble currents. A galvanoscope becomes agalvanometerby providing it with a scale so that the deflection may be measured.A galvanometer is really, in principle, an ammeter the scale of which has not been calibrated to read in amperes.Fig. 110.—Simple Compass Galvanoscope.Fig. 110.—Simple Compass Galvanoscope.A very simple galvanoscope may be made by winding fifty turns of No. 36 B. & S. gauge single-silk-covered wire around an ordinary pocket compass. The compass may be set in a block of wood, and the wood provided with binding-posts so that connections are easily made.Another variety of the same instrument is shown in Figure 111.Fig. 111.—Galvanoscope.Fig. 111.—Galvanoscope.Wind about twenty-five turns of No. 30 B. & S. gauge cotton-covered wire around the lower end of a glass tumbler. Leave about six inches of each end free for terminals, and then, after slipping the coil from the glass, tie the wire with thread in several places so that it will not unwind. Press two sides of the coil together so as to flatten it, and then attach it to a block of wood with some hot sealing-wax.Make a little wooden bridge as shown in Figure 111, and mount a compass-needle on it in the center. The compass-needle may be made out of a piece of spring-steel in the manner already described in Chapter I.Mount two binding-posts to the corners of the block, and connect the ends of the wire coil to them. Turn the block so that the needle points North and South and parallel to the coil of wire.If a battery is connected to the binding-posts, the needle will fly around to a position at right angles to that which it first occupied.An astatic galvanoscope is one having two needles with their poles in opposite directions. The word "astatic" means having no directive magnetic tendency. If the needles of an astatic pair are separated and pivoted separately, they will each point to North and South in the ordinary manner. But when connected together with the poles arranged in opposite directions they neutralize each other.An astatic needle requires but very little current in order to turn it either one way or the other, and for this reason an astatic galvanoscope is usually very sensitive.A simple instrument of this sort may be made by winding about fifty turns of No. 30-36 B. & S. gauge single-silk or cotton-insulated wire into a coil around a glass tumbler. After removing the coil from the glass, shape it into the form of an ellipse and fasten it to a small base-board.Separate the strands of wire at the top of the coil so that they are divided into two groups.Fig. 112.—Astatic Galvanoscope.Fig. 112.—Astatic Galvanoscope.Make a bridge or standard in the shape of an inverted U out of thin wooden strips and fasten it to the block.The needles are ordinary sewing-needles which have been magnetized and shoved through a small carrier-bar, made from a strip of cardboard, with their poles opposite one another, as shown in the illustration.Fig. 113.—Astatic Needles.Fig. 113.—Astatic Needles.They may be held in place in the cardboard strip by a small drop of sealing-wax.A small hole is punched in the top of the carrier, through which to pass the end of a thread. The upper end of the thread passes through a hole in the bridge and is tied to a small screw-eye in the center of the upper side of the bridge.The carrier-bar is passed through the space where the coil is split at the top. The lower needle should hang in the center of the coil. The upper needle should be above and outside the coil.The terminals of the coil are connected to two binding-posts mounted on the base-block.Owing to the fact that this galvanoscope is fitted with an astatic needle, the instrument does not have to be turned so that the coil may face North and South. The slightest current of electricity passing into the coil will instantly affect the needles.An astatic galvanometer for the detection of exceedingly weak currents and for use in connection with a "Wheatstone bridge" for measuring resistance, as described farther on, will form a valuable addition to the laboratory of the boy electrician.Make two small bobbins similar to those already described in connection with the volt and ammeter, but twice as long, as shown in Figure 114.Wind each of the bobbins in the same direction with No. 36 silk-covered or cotton-covered wire, leaving about six inches free at the ends for connection to the binding-posts.Fasten each of the bobbins to the base-board with glue. Do not nail or screw them in position, because the presence of nails or screws may impair the sensitiveness of the instrument. In mounting the bobbins, leave about one-sixteenth of an inch of space between the inside flanges, through which the needle may pass.Connect the coils wound on the bobbins so that the end of the outside layer of the first coil is connected to the inside layer of the other coil. This arrangement is so that the current will travel through the windings in the same continuous direction, exactly the same as though the bobbin were one continuous spool.Fig. 114.—Bobbin for Astatic Galvanometer.Fig. 114.—Bobbin for Astatic Galvanometer.Magnetize two small sewing-needles and mount them in a paper stirrup made from good, strong paper, as shown in Figure 114. Take care that the poles are reversed so that the north pole of one magnet will be on the same side of the stirrup as the south pole of the other. They may be fastened securely by a drop of shellac or melted sealing-wax.Cut out a cardboard disk and divide it into degrees as in Figure 115. Glue the disk to the top of the bobbins. A small slot should be cut in the disk so that it will pass the lower needle.A wooden post should be glued to the back of the base. To the top of this post is fastened an arm from which are suspended the magnetic needles.A fine fiber for suspending the needle may be secured by unraveling a piece of embroidery silk.Fig. 115.—Completed Astatic Galvanometer.Fig. 115.—Completed Astatic Galvanometer.The upper end of the fiber is tied to a small hook in the end of the arm. The wire hook may be twisted so that the needles may be brought to zero on the scale. Zero should lie on a line parallel to the two coils.The fiber used for suspending the needles should be as fine as possible. The finer the fiber is, the more sensitive will the instrument be.The lower needle should swing inside of the two coils, and the upper needle above the disk.How to Make a Wheatstone BridgeThe amateur experimenter will find many occasions when it is desirable to know the resistance of some of his electrical apparatus. Telephone receivers, telegraph relays, etc., are all graded according to their resistance in ohms. The measurement of resistance in any electrical instrument or circuit is usually accomplished by comparing its resistance with that of some known circuit, such as a coil of wire which has been previously tested.The simplest method of measuring resistance is by means of a device known as the Wheatstone bridge. This instrument is very simple but at the same time is remarkably sensitive if properly made. A Wheatstone bridge is shown in Figure 116.The base is a piece of well-seasoned hard wood, thirty inches long, six inches wide, and three-quarters of an inch thick.Secure a long strip of No. 18 B. & S. gauge sheet-copper, one inch wide, and cut it into three pieces, making two of the pieces three inches long, and the other piece twenty-three and one-half inches long.Mount the copper strips on the base, as shown, being very careful to make the distance between the inside edges of the end-pieces just twenty-five inches. The strips should be fastened to the base with small round-headed brass screws. Mount two binding-posts on each of the short strips in the positions shown in the illustration, and three on the long strip. These binding-posts should pass through the base and make firm contact with the strips.Fig. 116.—Wheatstone Bridge.Fig. 116.—Wheatstone Bridge.Then make a paper scale twenty-five inches long, and divide it into one hundred equal divisions one-quarter of an inch long. Mark every fifth division with a slightly longer line, and every tenth division with a double-length line.Start at one end and number every ten divisions, then start at the other end and number them back, so that the scale reads 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, from right to left at the top and 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, from left to right at the bottom.Solder a piece of No. 30 B. & S. gauge German-silver wire to one of the short copper strips opposite the end of the scale, and then stretch it tightly across the scale and solder it to the strip at the other end.Make a knife-contact by flattening a piece of heavy copper wire as shown in Figure 117. Solder a piece of flexible wire, such as "lamp cord," at the other end. It is well to fit the contact with a small wooden handle, made by boring out a piece of dowel.The instrument is now practically complete.Fig. 117.—Knife-Contact.Fig. 117.—Knife-Contact.In order to use the Wheatstone bridge, it is necessary to have a set of resistances of known value. The resistance of any unknown circuit or piece of apparatus is found by comparing it with one of the known coils. It is just like going to a store and buying a pound of sugar. The grocer weighs out the sugar by balancing it on the scales with an iron weight of known value, and taking it for granted that the weight is correct, we would say that we have one, five, or ten pounds of sugar, as the case may be.The Wheatstone bridge might be called a pair of "electrical scales" for weighing resistance by comparing an unknown coil with one which we know has a certain value.The next step is to make up some standard resistance coils. Secure some No. 32 B. & S. gauge single-cotton-covered wire from an electrical dealer and cut into the following lengths, laying it straight on the floor but using care not to pull or stretch it.1/2 ohm coil—3 feet 1/2 inch1 ohm coil—6 feet 1 1/4 inches2 ohm coil—12 feet 2 1/2 inches5 ohm coil—30 feet 6 1/4 inches10 ohm coil—61 feet20 ohm coil—122 feet30 ohm coil—183 feet50 ohm coil—305 feetThese lengths of wire are then wrapped on the spools in the following manner.Fig. 118.—Resistance-Coil.Fig. 118.—Resistance-Coil.Ashows how the Wire is doubled and wound on the Spool.Bis the completed Coil.This method of winding is known as the non-inductive method, because the windings do not generate a magnetic field, which might affect the galvanometer needle used in connection with the Wheatstone bridge as described later on.Each length of wire should be doubled exactly in the middle, then wrapped on the spools like a single wire, the two ends being left free for soldering to the terminals as shown in Figure 118, B.The spools may be the ordinary reels upon which cotton and sewing-silk are wrapped.The terminals of the spools are pieces of stout copper wire, No. 12 or No. 14 B. & S. gauge. Two pieces of wire about three inches long are driven into holes bored in the ends of each spool. A small drop of solder is used permanently to secure the ends of the coil to each of the heavy wire terminals.The spools are then dipped into a pan of molten paraffin and boiled until the air bubbles cease to rise.The spools should be marked 1, 2, 10, 20, 30, and 50, according to the amount of wire each one contains as indicated in the table above.How to Use a Wheatstone Bridge for Measuring ResistanceThe instrument is connected as in Figure 116.The unknown resistance or device to be measured is connected across the gap atB. One of the standard known coils is connected across the gap atA. A sensitive galvanometer or a telephone receiver and two cells of battery are also connected as shown.If a telephone receiver is used, place it to the ear. If a galvanometer is used instead, watch the needle carefully. Then move the sharp edge of the knife-contact over the scale along the German-silver "slide wire" until a point is reached when there is no deflection of the needle or no sound in the telephone receiver.If this point lies very far on one side or the other of the center division on the scale, substitute the next higher or lower known resistance spool until the point falls as near as possible to the center of the scale.When this point is found, note the reading on the scale carefully. Now comes the hardest part. Almost all my readers have no doubt progressed far enough in arithmetic to be able to carry on the following simple calculation in proportion which must be made in order to find out the resistance of the unknown coil.The unknown resistance, connected toB, bears the same ratio to the known coil, atA, that the number of divisions between the knife-contact and the right-hand end of the scale (lower row of figures) bears to the number of divisions between the knife-edge and the left-hand end of the scale (upper row of figures).We will suppose that a 5-ohm coil was used atAin a test, and the needle of the galvanometer stopped swinging when the knife-contact rested on the 60th division from the left-hand end, or on the 40th from the right. Then, in order to find the value of the unknown resistance atB, it is simply necessary to multiply the standard resistance atAby the number of left-hand divisions and divide the product by the number of right-hand divisions. The answer will be the resistance ofBin ohms.The calculation in this case would be as follows:5 X 40 = 200200/60 = 3.33 ohms3.33 ohms is the resistance ofB.This explanation may seem very long and complex, but if you will study it carefully you will find it to be very simple. When once you master it, you will be enabled to make many measurements of resistance which will add greatly to the interest and value of your experiments.BELLS, ALARMS, AND ANNUNCIATORS
CHAPTER VIII ELECTRICAL MEASURING INSTRUMENTSAn instrument designed to measure electromotive force (electrical pressure) is called avoltmeter. An instrument designed to measure volume of current is called anammeter.There are many forms of reliable meters for measuring current and voltage, but all are more or less expensive and out of the reach of an ordinary boy.Some meters are more carefully made than a watch, and are provided with fine hair-springs and jeweled bearings, but all depend upon the same principle for their action, namely, the mutual effects produced between a magnetic needle and a coil of insulated wire carrying a current of electricity.The little meters described in this chapter are simple and inexpensive but quite sensitive. Unlike a meter making use of a hair-spring, they will stand considerable rough handling, but of course should not be subjected to such treatment unnecessarily.Two types of meters are described. Both operate on exactly the same principle, but one is more elaborate than the other.A Simple Voltmeter and AmmeterA base-board five inches long, two and one-half inches wide and one-half inch thick is cut out of hard wood. In its center, cut a slot three-eighths of an inch wide and one and one-half inches long, with the slot running lengthwise the board. Along each side of the slot glue two small wooden blocks one and one-half inches long, one-quarter of an inch thick, and one-half of an inch high.Fig. 102.—*A*, Base, showing Slot. *B* and *C*, Sides and Top of the Bobbin. *D*, Base and Bobbin in Position.Fig. 102.—A, Base, showing Slot.BandC, Sides and Top of the Bobbin.D, Base and Bobbin in Position.When they are firmly in position, glue a strip of wood, two and one-half inches long, three-quarters of an inch wide and one-eighth inch thick to the top as shown by D in Figure 102.Using these as a support, wind a horizontal coil composed of 200 feet of No. 36 B. & S. gauge silk-covered wire.A needle is next made from a piece of watch-spring. It should be about one and one-quarter inches long, and one-eighth of an inch wide.Straighten it out by bending, and then heat the center in a small alcohol flame until the center is red-hot, taking care to keep the ends as cool as possible.The spring is mounted on a small steel shaft made by breaking up an ordinary sewing-needle. Make the piece one-half of an inch long. It must have very sharp points at both ends. The ends may be pointed by grinding.Fig. 103.—Arrangement of the Needle and Pointer.Fig. 103.—Arrangement of the Needle and Pointer.Bore a small hole just large enough to receive the needle through the center of the spring. Insert the needle in the hole and fasten it in the center by two small circular pieces of wood which fit tightly on the needle. A little glue or sealing-wax will serve to help make everything firm.The pointer is a piece of broom-straw, about three inches long. Bore a small hole in the top of one of the wooden clamps and insert the pointer in the hole, fastening it with a little glue. The pointer should be perfectly straight, and in a position at right angles to the spring.Bore a small hole in the bottom of one of the wooden clamps and glue a small wire nail in the hole. The purpose of the nail is to serve as a counterweight and keep the pointer in a vertical position.The spring should be magnetized by winding ten or twelve turns of magnet wire around one end and connecting it with a battery for a moment.Fig. 104.—A, Bearings. B, How the Needle is mounted.Fig. 104.—A, Bearings.B, How the Needle is mounted.The needle is mounted in two small pieces of thin sheet-brass, one inch long and one-half inch wide. Bend each strip at right angles in the middle, and at one-quarter of an inch from one end make a small dent by means of a pointed nail and a hammer.The strips are now slipped down in the center of the slot in the coil with the dents inside of the coil and exactly opposite one another. After the exact position is found, they may be fastened into position by two very small screws.The sharp-pointed sewing-needle, together with the magnetized spring, pointer, and counterweight, should slip down into the dents made in the strips and swing freely there. It may require a little filing and bending, but the work should be done patiently, because the proper working of the meter will depend upon having the needle swing freely and easily in its place.Fasten an upright board, four inches wide and one-quarter of an inch thick, to the base-board, back of the bobbin.Attach a piece of thick cardboard to the upright by means of small blocks, in such a position that the pointer swings very close to it but does not touch it.The meter is now complete, except for marking or calibrating the scale. The method of accomplishing this will be described farther on.Fig. 105.—The Completed Meter.Fig. 105.—The Completed Meter.If the meter is wound with No. 36 B. & S. gauge wire it is a voltmeter for measuring voltage. If it is wound with No. 16 B. & S. gauge wire it will constitute an ammeter for measuring amperes.A Portable Voltmeter and AmmeterThe bobbin upon which the wire is wound is illustrated in Figure 106. The wood is the Spanish cedar, of which cigar boxes are made. It should be one-eighth of an inch thick, and can be easily worked with a pocket-knife. In laying out the work, scratch the lines on the wood with the point of a darning-needle. Pencil lines are too thick to permit of accuracy in small work. The bobbin when finished must be perfectly true and square.The dimensions are best understood from the illustrations. In putting the bobbin together, do not use any nails. Use strong glue only.Two bobbins are required, one for the ammeter and one for the voltmeter. After completing the bobbins, sandpaper them and coat them with shellac.Fig. 106.—Details of the Bobbin.Fig. 106.—Details of the Bobbin.The bobbin for the ammeter is wound with No. 14 B. & S. double-cotton-covered magnet wire. The voltmeter requires No. 40 B. & S. silk-covered wire. In both cases the wire should be wound carefully in smooth, even layers. A small hole is bored in the flange through which to pass the end of the wire when starting the first layer. After finishing the winding, about six inches of wire should be left at both ends to make connection with the terminals. The whole winding is then given a coat of shellac. A strip of passe-partout tape, one-half of an inch wide wound over the wire around the bobbin will not only protect the wire from injury, but also give the bobbin a very neat appearance.The armature is a piece of soft steel one inch long, one-eighth of an inch thick and three-eighths wide. A one-eighth-inch hole is bored one-sixteenth of an inch above the center for the reception of the shaft. The center of gravity is thus thrown below the center of the mass of the armature, and the pointer will always return to zero if the instrument is level.The shaft is a piece of one-eighth-inch Bessemer steel rod, seven-sixteenths of an inch long. The ends are filed to a sharp knife-edge on the under side, as indicated in the figure.Fig. 107.—The Bobbin partly cut away so as to show the Bearing. Details of the Armature and Shaft.Fig. 107.—The Bobbin partly cut away so as to show the Bearing. Details of the Armature and Shaft.A one-sixteenth-inch hole is bored in the top of the armature to receive the lower end of the pointer, which is a piece of No. 16 aluminum wire, four and one-half inches long.After the holes have been bored, the armature is tempered so that it will retain its magnetism. It is heated to a bright red heat and dropped into a basin of strong salt water. The armature is then magnetized by rubbing one end against the pole of a strong magnet.The bearings are formed by two strips of thin sheet-brass, three-sixteenths of an inch wide, and one and one-quarter inches long, bent and glued to the sides of the bobbin.In the illustration, part of the bobbin is represented as cut away. The center of the bearing is bent out so that the end of the shaft will not come in contact with the sides of the bobbin. The top of the center is notched with a file to form a socket for the knife-edges of the shaft.Fig. 108.—Completed Voltmeter.Fig. 108.—Completed Voltmeter.The bobbin is glued to the center of a wooden base, seven inches long, four inches wide and three-quarters of an inch thick. The terminals of the coil lead down through two small holes in the base and thence to two large binding-posts. The wires are inlaid on the under side of the base, i.e., they pass from the holes to the binding-posts through two grooves. This precaution avoids the possibility of their becoming short-circuited or broken.The case is formed of two sides, a back and top of one-half-inch wood. It is six inches high, four inches wide, and two inches deep. A glass front slides in two shallow grooves cut in the wooden sides, one-eighth of an inch from the front.The case is held down to the base by four round-headed brass screws, which pass through the base into the sides. It is then easily removable in case it ever becomes necessary to repair or adjust the instrument.The meter and case, as illustrated in Figure 108, are intended for portable use and are so constructed that they will stand up. A small brass screw, long enough to pass all the way through the base, serves to level the instrument. If a little brass strip is placed in the slot in the screw-head and soldered so as to form what is known as a "winged screw," the adjustment may be made with the fingers and without the aid of a screw-driver.Where the instrument is intended for mounting upon a switch-board, it can be given a much better appearance by fitting with a smaller base, similar in size and shape to the top. The binding-posts are then mounted in the center of the sides.To calibrate the meters properly, they are compared with some standard. The scale is formed by a piece of white cardboard glued by two small blocks on the inside of the case. The various values are marked with a pen and ink. The glass front, therefore, cannot be put in place until they are located.The zero value on the meters will normally be in the center of the scale. When a current is passed through the bobbin, the armature tends to swing around at right angles to the turns of wire. But since the armature is pivoted above the center of the mass, when it swings, the center of gravity is displaced and exerts a pull in opposition to that of the bobbin, and the amount of swing indicated by the pointer will be greater as the current is stronger. The pointer will swing either to the right or the left, depending upon the direction in which the current passes through the bobbin. The pointer of the instrument illustrated in Figure 108 is at zero when at the extreme left of the scale. The pointer is bent to the left, so that the current will be registered when passing through the meter only in one direction, but the scale will have a greater range of values. It will also be necessary to cut a small groove in the base of the instrument in this case so that the armature will have plenty of room in which to swing.Fig. 109.—Circuits for Calibrating the Ammeter and Voltmeter.Fig. 109.—Circuits for Calibrating the Ammeter and Voltmeter.When calibrating the ammeter, it is placed in series with the standard meter, a set of strong batteries, and a rheostat. The rheostat is adjusted so that various current readings are obtained. The corresponding positions of the pointer on the meter being calibrated are then located for each value.The voltmeters must be placed in parallel, or shunt with each other, and in series with several battery cells. A switch is arranged so that the voltage of a varying number of cells may be passed through the meters. To secure fractional values of a volt, the rheostat is placed in shunt with the first cell of the battery. Then, by adjusting both the switch and the rheostat, any voltage within the maximum range of the battery may be secured.This means of regulating voltage is a common one, and of much use in wireless telegraph circuits, as will be explained later.When using the meters, it is always necessary that the ammeter shall be in series and the voltmeter in parallel or in shunt with the circuit.Galvanoscopes and GalvanometersIn the first part of Chapter V it was explained that several turns of wire surrounding a compass-needle would cause the needle to move and show a deflection if a current of electricity were sent through the coil.Such an instrument is called agalvanoscopeand may be used for detecting very feeble currents. A galvanoscope becomes agalvanometerby providing it with a scale so that the deflection may be measured.A galvanometer is really, in principle, an ammeter the scale of which has not been calibrated to read in amperes.Fig. 110.—Simple Compass Galvanoscope.Fig. 110.—Simple Compass Galvanoscope.A very simple galvanoscope may be made by winding fifty turns of No. 36 B. & S. gauge single-silk-covered wire around an ordinary pocket compass. The compass may be set in a block of wood, and the wood provided with binding-posts so that connections are easily made.Another variety of the same instrument is shown in Figure 111.Fig. 111.—Galvanoscope.Fig. 111.—Galvanoscope.Wind about twenty-five turns of No. 30 B. & S. gauge cotton-covered wire around the lower end of a glass tumbler. Leave about six inches of each end free for terminals, and then, after slipping the coil from the glass, tie the wire with thread in several places so that it will not unwind. Press two sides of the coil together so as to flatten it, and then attach it to a block of wood with some hot sealing-wax.Make a little wooden bridge as shown in Figure 111, and mount a compass-needle on it in the center. The compass-needle may be made out of a piece of spring-steel in the manner already described in Chapter I.Mount two binding-posts to the corners of the block, and connect the ends of the wire coil to them. Turn the block so that the needle points North and South and parallel to the coil of wire.If a battery is connected to the binding-posts, the needle will fly around to a position at right angles to that which it first occupied.An astatic galvanoscope is one having two needles with their poles in opposite directions. The word "astatic" means having no directive magnetic tendency. If the needles of an astatic pair are separated and pivoted separately, they will each point to North and South in the ordinary manner. But when connected together with the poles arranged in opposite directions they neutralize each other.An astatic needle requires but very little current in order to turn it either one way or the other, and for this reason an astatic galvanoscope is usually very sensitive.A simple instrument of this sort may be made by winding about fifty turns of No. 30-36 B. & S. gauge single-silk or cotton-insulated wire into a coil around a glass tumbler. After removing the coil from the glass, shape it into the form of an ellipse and fasten it to a small base-board.Separate the strands of wire at the top of the coil so that they are divided into two groups.Fig. 112.—Astatic Galvanoscope.Fig. 112.—Astatic Galvanoscope.Make a bridge or standard in the shape of an inverted U out of thin wooden strips and fasten it to the block.The needles are ordinary sewing-needles which have been magnetized and shoved through a small carrier-bar, made from a strip of cardboard, with their poles opposite one another, as shown in the illustration.Fig. 113.—Astatic Needles.Fig. 113.—Astatic Needles.They may be held in place in the cardboard strip by a small drop of sealing-wax.A small hole is punched in the top of the carrier, through which to pass the end of a thread. The upper end of the thread passes through a hole in the bridge and is tied to a small screw-eye in the center of the upper side of the bridge.The carrier-bar is passed through the space where the coil is split at the top. The lower needle should hang in the center of the coil. The upper needle should be above and outside the coil.The terminals of the coil are connected to two binding-posts mounted on the base-block.Owing to the fact that this galvanoscope is fitted with an astatic needle, the instrument does not have to be turned so that the coil may face North and South. The slightest current of electricity passing into the coil will instantly affect the needles.An astatic galvanometer for the detection of exceedingly weak currents and for use in connection with a "Wheatstone bridge" for measuring resistance, as described farther on, will form a valuable addition to the laboratory of the boy electrician.Make two small bobbins similar to those already described in connection with the volt and ammeter, but twice as long, as shown in Figure 114.Wind each of the bobbins in the same direction with No. 36 silk-covered or cotton-covered wire, leaving about six inches free at the ends for connection to the binding-posts.Fasten each of the bobbins to the base-board with glue. Do not nail or screw them in position, because the presence of nails or screws may impair the sensitiveness of the instrument. In mounting the bobbins, leave about one-sixteenth of an inch of space between the inside flanges, through which the needle may pass.Connect the coils wound on the bobbins so that the end of the outside layer of the first coil is connected to the inside layer of the other coil. This arrangement is so that the current will travel through the windings in the same continuous direction, exactly the same as though the bobbin were one continuous spool.Fig. 114.—Bobbin for Astatic Galvanometer.Fig. 114.—Bobbin for Astatic Galvanometer.Magnetize two small sewing-needles and mount them in a paper stirrup made from good, strong paper, as shown in Figure 114. Take care that the poles are reversed so that the north pole of one magnet will be on the same side of the stirrup as the south pole of the other. They may be fastened securely by a drop of shellac or melted sealing-wax.Cut out a cardboard disk and divide it into degrees as in Figure 115. Glue the disk to the top of the bobbins. A small slot should be cut in the disk so that it will pass the lower needle.A wooden post should be glued to the back of the base. To the top of this post is fastened an arm from which are suspended the magnetic needles.A fine fiber for suspending the needle may be secured by unraveling a piece of embroidery silk.Fig. 115.—Completed Astatic Galvanometer.Fig. 115.—Completed Astatic Galvanometer.The upper end of the fiber is tied to a small hook in the end of the arm. The wire hook may be twisted so that the needles may be brought to zero on the scale. Zero should lie on a line parallel to the two coils.The fiber used for suspending the needles should be as fine as possible. The finer the fiber is, the more sensitive will the instrument be.The lower needle should swing inside of the two coils, and the upper needle above the disk.How to Make a Wheatstone BridgeThe amateur experimenter will find many occasions when it is desirable to know the resistance of some of his electrical apparatus. Telephone receivers, telegraph relays, etc., are all graded according to their resistance in ohms. The measurement of resistance in any electrical instrument or circuit is usually accomplished by comparing its resistance with that of some known circuit, such as a coil of wire which has been previously tested.The simplest method of measuring resistance is by means of a device known as the Wheatstone bridge. This instrument is very simple but at the same time is remarkably sensitive if properly made. A Wheatstone bridge is shown in Figure 116.The base is a piece of well-seasoned hard wood, thirty inches long, six inches wide, and three-quarters of an inch thick.Secure a long strip of No. 18 B. & S. gauge sheet-copper, one inch wide, and cut it into three pieces, making two of the pieces three inches long, and the other piece twenty-three and one-half inches long.Mount the copper strips on the base, as shown, being very careful to make the distance between the inside edges of the end-pieces just twenty-five inches. The strips should be fastened to the base with small round-headed brass screws. Mount two binding-posts on each of the short strips in the positions shown in the illustration, and three on the long strip. These binding-posts should pass through the base and make firm contact with the strips.Fig. 116.—Wheatstone Bridge.Fig. 116.—Wheatstone Bridge.Then make a paper scale twenty-five inches long, and divide it into one hundred equal divisions one-quarter of an inch long. Mark every fifth division with a slightly longer line, and every tenth division with a double-length line.Start at one end and number every ten divisions, then start at the other end and number them back, so that the scale reads 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, from right to left at the top and 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, from left to right at the bottom.Solder a piece of No. 30 B. & S. gauge German-silver wire to one of the short copper strips opposite the end of the scale, and then stretch it tightly across the scale and solder it to the strip at the other end.Make a knife-contact by flattening a piece of heavy copper wire as shown in Figure 117. Solder a piece of flexible wire, such as "lamp cord," at the other end. It is well to fit the contact with a small wooden handle, made by boring out a piece of dowel.The instrument is now practically complete.Fig. 117.—Knife-Contact.Fig. 117.—Knife-Contact.In order to use the Wheatstone bridge, it is necessary to have a set of resistances of known value. The resistance of any unknown circuit or piece of apparatus is found by comparing it with one of the known coils. It is just like going to a store and buying a pound of sugar. The grocer weighs out the sugar by balancing it on the scales with an iron weight of known value, and taking it for granted that the weight is correct, we would say that we have one, five, or ten pounds of sugar, as the case may be.The Wheatstone bridge might be called a pair of "electrical scales" for weighing resistance by comparing an unknown coil with one which we know has a certain value.The next step is to make up some standard resistance coils. Secure some No. 32 B. & S. gauge single-cotton-covered wire from an electrical dealer and cut into the following lengths, laying it straight on the floor but using care not to pull or stretch it.1/2 ohm coil—3 feet 1/2 inch1 ohm coil—6 feet 1 1/4 inches2 ohm coil—12 feet 2 1/2 inches5 ohm coil—30 feet 6 1/4 inches10 ohm coil—61 feet20 ohm coil—122 feet30 ohm coil—183 feet50 ohm coil—305 feetThese lengths of wire are then wrapped on the spools in the following manner.Fig. 118.—Resistance-Coil.Fig. 118.—Resistance-Coil.Ashows how the Wire is doubled and wound on the Spool.Bis the completed Coil.This method of winding is known as the non-inductive method, because the windings do not generate a magnetic field, which might affect the galvanometer needle used in connection with the Wheatstone bridge as described later on.Each length of wire should be doubled exactly in the middle, then wrapped on the spools like a single wire, the two ends being left free for soldering to the terminals as shown in Figure 118, B.The spools may be the ordinary reels upon which cotton and sewing-silk are wrapped.The terminals of the spools are pieces of stout copper wire, No. 12 or No. 14 B. & S. gauge. Two pieces of wire about three inches long are driven into holes bored in the ends of each spool. A small drop of solder is used permanently to secure the ends of the coil to each of the heavy wire terminals.The spools are then dipped into a pan of molten paraffin and boiled until the air bubbles cease to rise.The spools should be marked 1, 2, 10, 20, 30, and 50, according to the amount of wire each one contains as indicated in the table above.How to Use a Wheatstone Bridge for Measuring ResistanceThe instrument is connected as in Figure 116.The unknown resistance or device to be measured is connected across the gap atB. One of the standard known coils is connected across the gap atA. A sensitive galvanometer or a telephone receiver and two cells of battery are also connected as shown.If a telephone receiver is used, place it to the ear. If a galvanometer is used instead, watch the needle carefully. Then move the sharp edge of the knife-contact over the scale along the German-silver "slide wire" until a point is reached when there is no deflection of the needle or no sound in the telephone receiver.If this point lies very far on one side or the other of the center division on the scale, substitute the next higher or lower known resistance spool until the point falls as near as possible to the center of the scale.When this point is found, note the reading on the scale carefully. Now comes the hardest part. Almost all my readers have no doubt progressed far enough in arithmetic to be able to carry on the following simple calculation in proportion which must be made in order to find out the resistance of the unknown coil.The unknown resistance, connected toB, bears the same ratio to the known coil, atA, that the number of divisions between the knife-contact and the right-hand end of the scale (lower row of figures) bears to the number of divisions between the knife-edge and the left-hand end of the scale (upper row of figures).We will suppose that a 5-ohm coil was used atAin a test, and the needle of the galvanometer stopped swinging when the knife-contact rested on the 60th division from the left-hand end, or on the 40th from the right. Then, in order to find the value of the unknown resistance atB, it is simply necessary to multiply the standard resistance atAby the number of left-hand divisions and divide the product by the number of right-hand divisions. The answer will be the resistance ofBin ohms.The calculation in this case would be as follows:5 X 40 = 200200/60 = 3.33 ohms3.33 ohms is the resistance ofB.This explanation may seem very long and complex, but if you will study it carefully you will find it to be very simple. When once you master it, you will be enabled to make many measurements of resistance which will add greatly to the interest and value of your experiments.BELLS, ALARMS, AND ANNUNCIATORS
An instrument designed to measure electromotive force (electrical pressure) is called avoltmeter. An instrument designed to measure volume of current is called anammeter.
There are many forms of reliable meters for measuring current and voltage, but all are more or less expensive and out of the reach of an ordinary boy.
Some meters are more carefully made than a watch, and are provided with fine hair-springs and jeweled bearings, but all depend upon the same principle for their action, namely, the mutual effects produced between a magnetic needle and a coil of insulated wire carrying a current of electricity.
The little meters described in this chapter are simple and inexpensive but quite sensitive. Unlike a meter making use of a hair-spring, they will stand considerable rough handling, but of course should not be subjected to such treatment unnecessarily.
Two types of meters are described. Both operate on exactly the same principle, but one is more elaborate than the other.
A Simple Voltmeter and AmmeterA base-board five inches long, two and one-half inches wide and one-half inch thick is cut out of hard wood. In its center, cut a slot three-eighths of an inch wide and one and one-half inches long, with the slot running lengthwise the board. Along each side of the slot glue two small wooden blocks one and one-half inches long, one-quarter of an inch thick, and one-half of an inch high.Fig. 102.—*A*, Base, showing Slot. *B* and *C*, Sides and Top of the Bobbin. *D*, Base and Bobbin in Position.Fig. 102.—A, Base, showing Slot.BandC, Sides and Top of the Bobbin.D, Base and Bobbin in Position.When they are firmly in position, glue a strip of wood, two and one-half inches long, three-quarters of an inch wide and one-eighth inch thick to the top as shown by D in Figure 102.Using these as a support, wind a horizontal coil composed of 200 feet of No. 36 B. & S. gauge silk-covered wire.A needle is next made from a piece of watch-spring. It should be about one and one-quarter inches long, and one-eighth of an inch wide.Straighten it out by bending, and then heat the center in a small alcohol flame until the center is red-hot, taking care to keep the ends as cool as possible.The spring is mounted on a small steel shaft made by breaking up an ordinary sewing-needle. Make the piece one-half of an inch long. It must have very sharp points at both ends. The ends may be pointed by grinding.Fig. 103.—Arrangement of the Needle and Pointer.Fig. 103.—Arrangement of the Needle and Pointer.Bore a small hole just large enough to receive the needle through the center of the spring. Insert the needle in the hole and fasten it in the center by two small circular pieces of wood which fit tightly on the needle. A little glue or sealing-wax will serve to help make everything firm.The pointer is a piece of broom-straw, about three inches long. Bore a small hole in the top of one of the wooden clamps and insert the pointer in the hole, fastening it with a little glue. The pointer should be perfectly straight, and in a position at right angles to the spring.Bore a small hole in the bottom of one of the wooden clamps and glue a small wire nail in the hole. The purpose of the nail is to serve as a counterweight and keep the pointer in a vertical position.The spring should be magnetized by winding ten or twelve turns of magnet wire around one end and connecting it with a battery for a moment.Fig. 104.—A, Bearings. B, How the Needle is mounted.Fig. 104.—A, Bearings.B, How the Needle is mounted.The needle is mounted in two small pieces of thin sheet-brass, one inch long and one-half inch wide. Bend each strip at right angles in the middle, and at one-quarter of an inch from one end make a small dent by means of a pointed nail and a hammer.The strips are now slipped down in the center of the slot in the coil with the dents inside of the coil and exactly opposite one another. After the exact position is found, they may be fastened into position by two very small screws.The sharp-pointed sewing-needle, together with the magnetized spring, pointer, and counterweight, should slip down into the dents made in the strips and swing freely there. It may require a little filing and bending, but the work should be done patiently, because the proper working of the meter will depend upon having the needle swing freely and easily in its place.Fasten an upright board, four inches wide and one-quarter of an inch thick, to the base-board, back of the bobbin.Attach a piece of thick cardboard to the upright by means of small blocks, in such a position that the pointer swings very close to it but does not touch it.The meter is now complete, except for marking or calibrating the scale. The method of accomplishing this will be described farther on.Fig. 105.—The Completed Meter.Fig. 105.—The Completed Meter.If the meter is wound with No. 36 B. & S. gauge wire it is a voltmeter for measuring voltage. If it is wound with No. 16 B. & S. gauge wire it will constitute an ammeter for measuring amperes.
A base-board five inches long, two and one-half inches wide and one-half inch thick is cut out of hard wood. In its center, cut a slot three-eighths of an inch wide and one and one-half inches long, with the slot running lengthwise the board. Along each side of the slot glue two small wooden blocks one and one-half inches long, one-quarter of an inch thick, and one-half of an inch high.
Fig. 102.—*A*, Base, showing Slot. *B* and *C*, Sides and Top of the Bobbin. *D*, Base and Bobbin in Position.Fig. 102.—A, Base, showing Slot.BandC, Sides and Top of the Bobbin.D, Base and Bobbin in Position.
Fig. 102.—A, Base, showing Slot.BandC, Sides and Top of the Bobbin.D, Base and Bobbin in Position.
When they are firmly in position, glue a strip of wood, two and one-half inches long, three-quarters of an inch wide and one-eighth inch thick to the top as shown by D in Figure 102.
Using these as a support, wind a horizontal coil composed of 200 feet of No. 36 B. & S. gauge silk-covered wire.
A needle is next made from a piece of watch-spring. It should be about one and one-quarter inches long, and one-eighth of an inch wide.
Straighten it out by bending, and then heat the center in a small alcohol flame until the center is red-hot, taking care to keep the ends as cool as possible.
The spring is mounted on a small steel shaft made by breaking up an ordinary sewing-needle. Make the piece one-half of an inch long. It must have very sharp points at both ends. The ends may be pointed by grinding.
Fig. 103.—Arrangement of the Needle and Pointer.Fig. 103.—Arrangement of the Needle and Pointer.
Fig. 103.—Arrangement of the Needle and Pointer.
Bore a small hole just large enough to receive the needle through the center of the spring. Insert the needle in the hole and fasten it in the center by two small circular pieces of wood which fit tightly on the needle. A little glue or sealing-wax will serve to help make everything firm.
The pointer is a piece of broom-straw, about three inches long. Bore a small hole in the top of one of the wooden clamps and insert the pointer in the hole, fastening it with a little glue. The pointer should be perfectly straight, and in a position at right angles to the spring.
Bore a small hole in the bottom of one of the wooden clamps and glue a small wire nail in the hole. The purpose of the nail is to serve as a counterweight and keep the pointer in a vertical position.
The spring should be magnetized by winding ten or twelve turns of magnet wire around one end and connecting it with a battery for a moment.
Fig. 104.—A, Bearings. B, How the Needle is mounted.Fig. 104.—A, Bearings.B, How the Needle is mounted.
Fig. 104.—A, Bearings.B, How the Needle is mounted.
The needle is mounted in two small pieces of thin sheet-brass, one inch long and one-half inch wide. Bend each strip at right angles in the middle, and at one-quarter of an inch from one end make a small dent by means of a pointed nail and a hammer.
The strips are now slipped down in the center of the slot in the coil with the dents inside of the coil and exactly opposite one another. After the exact position is found, they may be fastened into position by two very small screws.
The sharp-pointed sewing-needle, together with the magnetized spring, pointer, and counterweight, should slip down into the dents made in the strips and swing freely there. It may require a little filing and bending, but the work should be done patiently, because the proper working of the meter will depend upon having the needle swing freely and easily in its place.
Fasten an upright board, four inches wide and one-quarter of an inch thick, to the base-board, back of the bobbin.
Attach a piece of thick cardboard to the upright by means of small blocks, in such a position that the pointer swings very close to it but does not touch it.
The meter is now complete, except for marking or calibrating the scale. The method of accomplishing this will be described farther on.
Fig. 105.—The Completed Meter.Fig. 105.—The Completed Meter.
Fig. 105.—The Completed Meter.
If the meter is wound with No. 36 B. & S. gauge wire it is a voltmeter for measuring voltage. If it is wound with No. 16 B. & S. gauge wire it will constitute an ammeter for measuring amperes.
A Portable Voltmeter and AmmeterThe bobbin upon which the wire is wound is illustrated in Figure 106. The wood is the Spanish cedar, of which cigar boxes are made. It should be one-eighth of an inch thick, and can be easily worked with a pocket-knife. In laying out the work, scratch the lines on the wood with the point of a darning-needle. Pencil lines are too thick to permit of accuracy in small work. The bobbin when finished must be perfectly true and square.The dimensions are best understood from the illustrations. In putting the bobbin together, do not use any nails. Use strong glue only.Two bobbins are required, one for the ammeter and one for the voltmeter. After completing the bobbins, sandpaper them and coat them with shellac.Fig. 106.—Details of the Bobbin.Fig. 106.—Details of the Bobbin.The bobbin for the ammeter is wound with No. 14 B. & S. double-cotton-covered magnet wire. The voltmeter requires No. 40 B. & S. silk-covered wire. In both cases the wire should be wound carefully in smooth, even layers. A small hole is bored in the flange through which to pass the end of the wire when starting the first layer. After finishing the winding, about six inches of wire should be left at both ends to make connection with the terminals. The whole winding is then given a coat of shellac. A strip of passe-partout tape, one-half of an inch wide wound over the wire around the bobbin will not only protect the wire from injury, but also give the bobbin a very neat appearance.The armature is a piece of soft steel one inch long, one-eighth of an inch thick and three-eighths wide. A one-eighth-inch hole is bored one-sixteenth of an inch above the center for the reception of the shaft. The center of gravity is thus thrown below the center of the mass of the armature, and the pointer will always return to zero if the instrument is level.The shaft is a piece of one-eighth-inch Bessemer steel rod, seven-sixteenths of an inch long. The ends are filed to a sharp knife-edge on the under side, as indicated in the figure.Fig. 107.—The Bobbin partly cut away so as to show the Bearing. Details of the Armature and Shaft.Fig. 107.—The Bobbin partly cut away so as to show the Bearing. Details of the Armature and Shaft.A one-sixteenth-inch hole is bored in the top of the armature to receive the lower end of the pointer, which is a piece of No. 16 aluminum wire, four and one-half inches long.After the holes have been bored, the armature is tempered so that it will retain its magnetism. It is heated to a bright red heat and dropped into a basin of strong salt water. The armature is then magnetized by rubbing one end against the pole of a strong magnet.The bearings are formed by two strips of thin sheet-brass, three-sixteenths of an inch wide, and one and one-quarter inches long, bent and glued to the sides of the bobbin.In the illustration, part of the bobbin is represented as cut away. The center of the bearing is bent out so that the end of the shaft will not come in contact with the sides of the bobbin. The top of the center is notched with a file to form a socket for the knife-edges of the shaft.Fig. 108.—Completed Voltmeter.Fig. 108.—Completed Voltmeter.The bobbin is glued to the center of a wooden base, seven inches long, four inches wide and three-quarters of an inch thick. The terminals of the coil lead down through two small holes in the base and thence to two large binding-posts. The wires are inlaid on the under side of the base, i.e., they pass from the holes to the binding-posts through two grooves. This precaution avoids the possibility of their becoming short-circuited or broken.The case is formed of two sides, a back and top of one-half-inch wood. It is six inches high, four inches wide, and two inches deep. A glass front slides in two shallow grooves cut in the wooden sides, one-eighth of an inch from the front.The case is held down to the base by four round-headed brass screws, which pass through the base into the sides. It is then easily removable in case it ever becomes necessary to repair or adjust the instrument.The meter and case, as illustrated in Figure 108, are intended for portable use and are so constructed that they will stand up. A small brass screw, long enough to pass all the way through the base, serves to level the instrument. If a little brass strip is placed in the slot in the screw-head and soldered so as to form what is known as a "winged screw," the adjustment may be made with the fingers and without the aid of a screw-driver.Where the instrument is intended for mounting upon a switch-board, it can be given a much better appearance by fitting with a smaller base, similar in size and shape to the top. The binding-posts are then mounted in the center of the sides.To calibrate the meters properly, they are compared with some standard. The scale is formed by a piece of white cardboard glued by two small blocks on the inside of the case. The various values are marked with a pen and ink. The glass front, therefore, cannot be put in place until they are located.The zero value on the meters will normally be in the center of the scale. When a current is passed through the bobbin, the armature tends to swing around at right angles to the turns of wire. But since the armature is pivoted above the center of the mass, when it swings, the center of gravity is displaced and exerts a pull in opposition to that of the bobbin, and the amount of swing indicated by the pointer will be greater as the current is stronger. The pointer will swing either to the right or the left, depending upon the direction in which the current passes through the bobbin. The pointer of the instrument illustrated in Figure 108 is at zero when at the extreme left of the scale. The pointer is bent to the left, so that the current will be registered when passing through the meter only in one direction, but the scale will have a greater range of values. It will also be necessary to cut a small groove in the base of the instrument in this case so that the armature will have plenty of room in which to swing.Fig. 109.—Circuits for Calibrating the Ammeter and Voltmeter.Fig. 109.—Circuits for Calibrating the Ammeter and Voltmeter.When calibrating the ammeter, it is placed in series with the standard meter, a set of strong batteries, and a rheostat. The rheostat is adjusted so that various current readings are obtained. The corresponding positions of the pointer on the meter being calibrated are then located for each value.The voltmeters must be placed in parallel, or shunt with each other, and in series with several battery cells. A switch is arranged so that the voltage of a varying number of cells may be passed through the meters. To secure fractional values of a volt, the rheostat is placed in shunt with the first cell of the battery. Then, by adjusting both the switch and the rheostat, any voltage within the maximum range of the battery may be secured.This means of regulating voltage is a common one, and of much use in wireless telegraph circuits, as will be explained later.When using the meters, it is always necessary that the ammeter shall be in series and the voltmeter in parallel or in shunt with the circuit.
The bobbin upon which the wire is wound is illustrated in Figure 106. The wood is the Spanish cedar, of which cigar boxes are made. It should be one-eighth of an inch thick, and can be easily worked with a pocket-knife. In laying out the work, scratch the lines on the wood with the point of a darning-needle. Pencil lines are too thick to permit of accuracy in small work. The bobbin when finished must be perfectly true and square.
The dimensions are best understood from the illustrations. In putting the bobbin together, do not use any nails. Use strong glue only.
Two bobbins are required, one for the ammeter and one for the voltmeter. After completing the bobbins, sandpaper them and coat them with shellac.
Fig. 106.—Details of the Bobbin.Fig. 106.—Details of the Bobbin.
Fig. 106.—Details of the Bobbin.
The bobbin for the ammeter is wound with No. 14 B. & S. double-cotton-covered magnet wire. The voltmeter requires No. 40 B. & S. silk-covered wire. In both cases the wire should be wound carefully in smooth, even layers. A small hole is bored in the flange through which to pass the end of the wire when starting the first layer. After finishing the winding, about six inches of wire should be left at both ends to make connection with the terminals. The whole winding is then given a coat of shellac. A strip of passe-partout tape, one-half of an inch wide wound over the wire around the bobbin will not only protect the wire from injury, but also give the bobbin a very neat appearance.
The armature is a piece of soft steel one inch long, one-eighth of an inch thick and three-eighths wide. A one-eighth-inch hole is bored one-sixteenth of an inch above the center for the reception of the shaft. The center of gravity is thus thrown below the center of the mass of the armature, and the pointer will always return to zero if the instrument is level.
The shaft is a piece of one-eighth-inch Bessemer steel rod, seven-sixteenths of an inch long. The ends are filed to a sharp knife-edge on the under side, as indicated in the figure.
Fig. 107.—The Bobbin partly cut away so as to show the Bearing. Details of the Armature and Shaft.Fig. 107.—The Bobbin partly cut away so as to show the Bearing. Details of the Armature and Shaft.
Fig. 107.—The Bobbin partly cut away so as to show the Bearing. Details of the Armature and Shaft.
A one-sixteenth-inch hole is bored in the top of the armature to receive the lower end of the pointer, which is a piece of No. 16 aluminum wire, four and one-half inches long.
After the holes have been bored, the armature is tempered so that it will retain its magnetism. It is heated to a bright red heat and dropped into a basin of strong salt water. The armature is then magnetized by rubbing one end against the pole of a strong magnet.
The bearings are formed by two strips of thin sheet-brass, three-sixteenths of an inch wide, and one and one-quarter inches long, bent and glued to the sides of the bobbin.
In the illustration, part of the bobbin is represented as cut away. The center of the bearing is bent out so that the end of the shaft will not come in contact with the sides of the bobbin. The top of the center is notched with a file to form a socket for the knife-edges of the shaft.
Fig. 108.—Completed Voltmeter.Fig. 108.—Completed Voltmeter.
Fig. 108.—Completed Voltmeter.
The bobbin is glued to the center of a wooden base, seven inches long, four inches wide and three-quarters of an inch thick. The terminals of the coil lead down through two small holes in the base and thence to two large binding-posts. The wires are inlaid on the under side of the base, i.e., they pass from the holes to the binding-posts through two grooves. This precaution avoids the possibility of their becoming short-circuited or broken.
The case is formed of two sides, a back and top of one-half-inch wood. It is six inches high, four inches wide, and two inches deep. A glass front slides in two shallow grooves cut in the wooden sides, one-eighth of an inch from the front.
The case is held down to the base by four round-headed brass screws, which pass through the base into the sides. It is then easily removable in case it ever becomes necessary to repair or adjust the instrument.
The meter and case, as illustrated in Figure 108, are intended for portable use and are so constructed that they will stand up. A small brass screw, long enough to pass all the way through the base, serves to level the instrument. If a little brass strip is placed in the slot in the screw-head and soldered so as to form what is known as a "winged screw," the adjustment may be made with the fingers and without the aid of a screw-driver.
Where the instrument is intended for mounting upon a switch-board, it can be given a much better appearance by fitting with a smaller base, similar in size and shape to the top. The binding-posts are then mounted in the center of the sides.
To calibrate the meters properly, they are compared with some standard. The scale is formed by a piece of white cardboard glued by two small blocks on the inside of the case. The various values are marked with a pen and ink. The glass front, therefore, cannot be put in place until they are located.
The zero value on the meters will normally be in the center of the scale. When a current is passed through the bobbin, the armature tends to swing around at right angles to the turns of wire. But since the armature is pivoted above the center of the mass, when it swings, the center of gravity is displaced and exerts a pull in opposition to that of the bobbin, and the amount of swing indicated by the pointer will be greater as the current is stronger. The pointer will swing either to the right or the left, depending upon the direction in which the current passes through the bobbin. The pointer of the instrument illustrated in Figure 108 is at zero when at the extreme left of the scale. The pointer is bent to the left, so that the current will be registered when passing through the meter only in one direction, but the scale will have a greater range of values. It will also be necessary to cut a small groove in the base of the instrument in this case so that the armature will have plenty of room in which to swing.
Fig. 109.—Circuits for Calibrating the Ammeter and Voltmeter.Fig. 109.—Circuits for Calibrating the Ammeter and Voltmeter.
Fig. 109.—Circuits for Calibrating the Ammeter and Voltmeter.
When calibrating the ammeter, it is placed in series with the standard meter, a set of strong batteries, and a rheostat. The rheostat is adjusted so that various current readings are obtained. The corresponding positions of the pointer on the meter being calibrated are then located for each value.
The voltmeters must be placed in parallel, or shunt with each other, and in series with several battery cells. A switch is arranged so that the voltage of a varying number of cells may be passed through the meters. To secure fractional values of a volt, the rheostat is placed in shunt with the first cell of the battery. Then, by adjusting both the switch and the rheostat, any voltage within the maximum range of the battery may be secured.
This means of regulating voltage is a common one, and of much use in wireless telegraph circuits, as will be explained later.
When using the meters, it is always necessary that the ammeter shall be in series and the voltmeter in parallel or in shunt with the circuit.
Galvanoscopes and GalvanometersIn the first part of Chapter V it was explained that several turns of wire surrounding a compass-needle would cause the needle to move and show a deflection if a current of electricity were sent through the coil.Such an instrument is called agalvanoscopeand may be used for detecting very feeble currents. A galvanoscope becomes agalvanometerby providing it with a scale so that the deflection may be measured.A galvanometer is really, in principle, an ammeter the scale of which has not been calibrated to read in amperes.Fig. 110.—Simple Compass Galvanoscope.Fig. 110.—Simple Compass Galvanoscope.A very simple galvanoscope may be made by winding fifty turns of No. 36 B. & S. gauge single-silk-covered wire around an ordinary pocket compass. The compass may be set in a block of wood, and the wood provided with binding-posts so that connections are easily made.Another variety of the same instrument is shown in Figure 111.Fig. 111.—Galvanoscope.Fig. 111.—Galvanoscope.Wind about twenty-five turns of No. 30 B. & S. gauge cotton-covered wire around the lower end of a glass tumbler. Leave about six inches of each end free for terminals, and then, after slipping the coil from the glass, tie the wire with thread in several places so that it will not unwind. Press two sides of the coil together so as to flatten it, and then attach it to a block of wood with some hot sealing-wax.Make a little wooden bridge as shown in Figure 111, and mount a compass-needle on it in the center. The compass-needle may be made out of a piece of spring-steel in the manner already described in Chapter I.Mount two binding-posts to the corners of the block, and connect the ends of the wire coil to them. Turn the block so that the needle points North and South and parallel to the coil of wire.If a battery is connected to the binding-posts, the needle will fly around to a position at right angles to that which it first occupied.An astatic galvanoscope is one having two needles with their poles in opposite directions. The word "astatic" means having no directive magnetic tendency. If the needles of an astatic pair are separated and pivoted separately, they will each point to North and South in the ordinary manner. But when connected together with the poles arranged in opposite directions they neutralize each other.An astatic needle requires but very little current in order to turn it either one way or the other, and for this reason an astatic galvanoscope is usually very sensitive.A simple instrument of this sort may be made by winding about fifty turns of No. 30-36 B. & S. gauge single-silk or cotton-insulated wire into a coil around a glass tumbler. After removing the coil from the glass, shape it into the form of an ellipse and fasten it to a small base-board.Separate the strands of wire at the top of the coil so that they are divided into two groups.Fig. 112.—Astatic Galvanoscope.Fig. 112.—Astatic Galvanoscope.Make a bridge or standard in the shape of an inverted U out of thin wooden strips and fasten it to the block.The needles are ordinary sewing-needles which have been magnetized and shoved through a small carrier-bar, made from a strip of cardboard, with their poles opposite one another, as shown in the illustration.Fig. 113.—Astatic Needles.Fig. 113.—Astatic Needles.They may be held in place in the cardboard strip by a small drop of sealing-wax.A small hole is punched in the top of the carrier, through which to pass the end of a thread. The upper end of the thread passes through a hole in the bridge and is tied to a small screw-eye in the center of the upper side of the bridge.The carrier-bar is passed through the space where the coil is split at the top. The lower needle should hang in the center of the coil. The upper needle should be above and outside the coil.The terminals of the coil are connected to two binding-posts mounted on the base-block.Owing to the fact that this galvanoscope is fitted with an astatic needle, the instrument does not have to be turned so that the coil may face North and South. The slightest current of electricity passing into the coil will instantly affect the needles.An astatic galvanometer for the detection of exceedingly weak currents and for use in connection with a "Wheatstone bridge" for measuring resistance, as described farther on, will form a valuable addition to the laboratory of the boy electrician.Make two small bobbins similar to those already described in connection with the volt and ammeter, but twice as long, as shown in Figure 114.Wind each of the bobbins in the same direction with No. 36 silk-covered or cotton-covered wire, leaving about six inches free at the ends for connection to the binding-posts.Fasten each of the bobbins to the base-board with glue. Do not nail or screw them in position, because the presence of nails or screws may impair the sensitiveness of the instrument. In mounting the bobbins, leave about one-sixteenth of an inch of space between the inside flanges, through which the needle may pass.Connect the coils wound on the bobbins so that the end of the outside layer of the first coil is connected to the inside layer of the other coil. This arrangement is so that the current will travel through the windings in the same continuous direction, exactly the same as though the bobbin were one continuous spool.Fig. 114.—Bobbin for Astatic Galvanometer.Fig. 114.—Bobbin for Astatic Galvanometer.Magnetize two small sewing-needles and mount them in a paper stirrup made from good, strong paper, as shown in Figure 114. Take care that the poles are reversed so that the north pole of one magnet will be on the same side of the stirrup as the south pole of the other. They may be fastened securely by a drop of shellac or melted sealing-wax.Cut out a cardboard disk and divide it into degrees as in Figure 115. Glue the disk to the top of the bobbins. A small slot should be cut in the disk so that it will pass the lower needle.A wooden post should be glued to the back of the base. To the top of this post is fastened an arm from which are suspended the magnetic needles.A fine fiber for suspending the needle may be secured by unraveling a piece of embroidery silk.Fig. 115.—Completed Astatic Galvanometer.Fig. 115.—Completed Astatic Galvanometer.The upper end of the fiber is tied to a small hook in the end of the arm. The wire hook may be twisted so that the needles may be brought to zero on the scale. Zero should lie on a line parallel to the two coils.The fiber used for suspending the needles should be as fine as possible. The finer the fiber is, the more sensitive will the instrument be.The lower needle should swing inside of the two coils, and the upper needle above the disk.
In the first part of Chapter V it was explained that several turns of wire surrounding a compass-needle would cause the needle to move and show a deflection if a current of electricity were sent through the coil.
Such an instrument is called agalvanoscopeand may be used for detecting very feeble currents. A galvanoscope becomes agalvanometerby providing it with a scale so that the deflection may be measured.
A galvanometer is really, in principle, an ammeter the scale of which has not been calibrated to read in amperes.
Fig. 110.—Simple Compass Galvanoscope.Fig. 110.—Simple Compass Galvanoscope.
Fig. 110.—Simple Compass Galvanoscope.
A very simple galvanoscope may be made by winding fifty turns of No. 36 B. & S. gauge single-silk-covered wire around an ordinary pocket compass. The compass may be set in a block of wood, and the wood provided with binding-posts so that connections are easily made.
Another variety of the same instrument is shown in Figure 111.
Fig. 111.—Galvanoscope.Fig. 111.—Galvanoscope.
Fig. 111.—Galvanoscope.
Wind about twenty-five turns of No. 30 B. & S. gauge cotton-covered wire around the lower end of a glass tumbler. Leave about six inches of each end free for terminals, and then, after slipping the coil from the glass, tie the wire with thread in several places so that it will not unwind. Press two sides of the coil together so as to flatten it, and then attach it to a block of wood with some hot sealing-wax.
Make a little wooden bridge as shown in Figure 111, and mount a compass-needle on it in the center. The compass-needle may be made out of a piece of spring-steel in the manner already described in Chapter I.
Mount two binding-posts to the corners of the block, and connect the ends of the wire coil to them. Turn the block so that the needle points North and South and parallel to the coil of wire.
If a battery is connected to the binding-posts, the needle will fly around to a position at right angles to that which it first occupied.
An astatic galvanoscope is one having two needles with their poles in opposite directions. The word "astatic" means having no directive magnetic tendency. If the needles of an astatic pair are separated and pivoted separately, they will each point to North and South in the ordinary manner. But when connected together with the poles arranged in opposite directions they neutralize each other.
An astatic needle requires but very little current in order to turn it either one way or the other, and for this reason an astatic galvanoscope is usually very sensitive.
A simple instrument of this sort may be made by winding about fifty turns of No. 30-36 B. & S. gauge single-silk or cotton-insulated wire into a coil around a glass tumbler. After removing the coil from the glass, shape it into the form of an ellipse and fasten it to a small base-board.
Separate the strands of wire at the top of the coil so that they are divided into two groups.
Fig. 112.—Astatic Galvanoscope.Fig. 112.—Astatic Galvanoscope.
Fig. 112.—Astatic Galvanoscope.
Make a bridge or standard in the shape of an inverted U out of thin wooden strips and fasten it to the block.
The needles are ordinary sewing-needles which have been magnetized and shoved through a small carrier-bar, made from a strip of cardboard, with their poles opposite one another, as shown in the illustration.
Fig. 113.—Astatic Needles.Fig. 113.—Astatic Needles.
Fig. 113.—Astatic Needles.
They may be held in place in the cardboard strip by a small drop of sealing-wax.
A small hole is punched in the top of the carrier, through which to pass the end of a thread. The upper end of the thread passes through a hole in the bridge and is tied to a small screw-eye in the center of the upper side of the bridge.
The carrier-bar is passed through the space where the coil is split at the top. The lower needle should hang in the center of the coil. The upper needle should be above and outside the coil.
The terminals of the coil are connected to two binding-posts mounted on the base-block.
Owing to the fact that this galvanoscope is fitted with an astatic needle, the instrument does not have to be turned so that the coil may face North and South. The slightest current of electricity passing into the coil will instantly affect the needles.
An astatic galvanometer for the detection of exceedingly weak currents and for use in connection with a "Wheatstone bridge" for measuring resistance, as described farther on, will form a valuable addition to the laboratory of the boy electrician.
Make two small bobbins similar to those already described in connection with the volt and ammeter, but twice as long, as shown in Figure 114.
Wind each of the bobbins in the same direction with No. 36 silk-covered or cotton-covered wire, leaving about six inches free at the ends for connection to the binding-posts.
Fasten each of the bobbins to the base-board with glue. Do not nail or screw them in position, because the presence of nails or screws may impair the sensitiveness of the instrument. In mounting the bobbins, leave about one-sixteenth of an inch of space between the inside flanges, through which the needle may pass.
Connect the coils wound on the bobbins so that the end of the outside layer of the first coil is connected to the inside layer of the other coil. This arrangement is so that the current will travel through the windings in the same continuous direction, exactly the same as though the bobbin were one continuous spool.
Fig. 114.—Bobbin for Astatic Galvanometer.Fig. 114.—Bobbin for Astatic Galvanometer.
Fig. 114.—Bobbin for Astatic Galvanometer.
Magnetize two small sewing-needles and mount them in a paper stirrup made from good, strong paper, as shown in Figure 114. Take care that the poles are reversed so that the north pole of one magnet will be on the same side of the stirrup as the south pole of the other. They may be fastened securely by a drop of shellac or melted sealing-wax.
Cut out a cardboard disk and divide it into degrees as in Figure 115. Glue the disk to the top of the bobbins. A small slot should be cut in the disk so that it will pass the lower needle.
A wooden post should be glued to the back of the base. To the top of this post is fastened an arm from which are suspended the magnetic needles.
A fine fiber for suspending the needle may be secured by unraveling a piece of embroidery silk.
Fig. 115.—Completed Astatic Galvanometer.Fig. 115.—Completed Astatic Galvanometer.
Fig. 115.—Completed Astatic Galvanometer.
The upper end of the fiber is tied to a small hook in the end of the arm. The wire hook may be twisted so that the needles may be brought to zero on the scale. Zero should lie on a line parallel to the two coils.
The fiber used for suspending the needles should be as fine as possible. The finer the fiber is, the more sensitive will the instrument be.
The lower needle should swing inside of the two coils, and the upper needle above the disk.
How to Make a Wheatstone BridgeThe amateur experimenter will find many occasions when it is desirable to know the resistance of some of his electrical apparatus. Telephone receivers, telegraph relays, etc., are all graded according to their resistance in ohms. The measurement of resistance in any electrical instrument or circuit is usually accomplished by comparing its resistance with that of some known circuit, such as a coil of wire which has been previously tested.The simplest method of measuring resistance is by means of a device known as the Wheatstone bridge. This instrument is very simple but at the same time is remarkably sensitive if properly made. A Wheatstone bridge is shown in Figure 116.The base is a piece of well-seasoned hard wood, thirty inches long, six inches wide, and three-quarters of an inch thick.Secure a long strip of No. 18 B. & S. gauge sheet-copper, one inch wide, and cut it into three pieces, making two of the pieces three inches long, and the other piece twenty-three and one-half inches long.Mount the copper strips on the base, as shown, being very careful to make the distance between the inside edges of the end-pieces just twenty-five inches. The strips should be fastened to the base with small round-headed brass screws. Mount two binding-posts on each of the short strips in the positions shown in the illustration, and three on the long strip. These binding-posts should pass through the base and make firm contact with the strips.Fig. 116.—Wheatstone Bridge.Fig. 116.—Wheatstone Bridge.Then make a paper scale twenty-five inches long, and divide it into one hundred equal divisions one-quarter of an inch long. Mark every fifth division with a slightly longer line, and every tenth division with a double-length line.Start at one end and number every ten divisions, then start at the other end and number them back, so that the scale reads 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, from right to left at the top and 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, from left to right at the bottom.Solder a piece of No. 30 B. & S. gauge German-silver wire to one of the short copper strips opposite the end of the scale, and then stretch it tightly across the scale and solder it to the strip at the other end.Make a knife-contact by flattening a piece of heavy copper wire as shown in Figure 117. Solder a piece of flexible wire, such as "lamp cord," at the other end. It is well to fit the contact with a small wooden handle, made by boring out a piece of dowel.The instrument is now practically complete.Fig. 117.—Knife-Contact.Fig. 117.—Knife-Contact.In order to use the Wheatstone bridge, it is necessary to have a set of resistances of known value. The resistance of any unknown circuit or piece of apparatus is found by comparing it with one of the known coils. It is just like going to a store and buying a pound of sugar. The grocer weighs out the sugar by balancing it on the scales with an iron weight of known value, and taking it for granted that the weight is correct, we would say that we have one, five, or ten pounds of sugar, as the case may be.The Wheatstone bridge might be called a pair of "electrical scales" for weighing resistance by comparing an unknown coil with one which we know has a certain value.The next step is to make up some standard resistance coils. Secure some No. 32 B. & S. gauge single-cotton-covered wire from an electrical dealer and cut into the following lengths, laying it straight on the floor but using care not to pull or stretch it.1/2 ohm coil—3 feet 1/2 inch1 ohm coil—6 feet 1 1/4 inches2 ohm coil—12 feet 2 1/2 inches5 ohm coil—30 feet 6 1/4 inches10 ohm coil—61 feet20 ohm coil—122 feet30 ohm coil—183 feet50 ohm coil—305 feetThese lengths of wire are then wrapped on the spools in the following manner.Fig. 118.—Resistance-Coil.Fig. 118.—Resistance-Coil.Ashows how the Wire is doubled and wound on the Spool.Bis the completed Coil.This method of winding is known as the non-inductive method, because the windings do not generate a magnetic field, which might affect the galvanometer needle used in connection with the Wheatstone bridge as described later on.Each length of wire should be doubled exactly in the middle, then wrapped on the spools like a single wire, the two ends being left free for soldering to the terminals as shown in Figure 118, B.The spools may be the ordinary reels upon which cotton and sewing-silk are wrapped.The terminals of the spools are pieces of stout copper wire, No. 12 or No. 14 B. & S. gauge. Two pieces of wire about three inches long are driven into holes bored in the ends of each spool. A small drop of solder is used permanently to secure the ends of the coil to each of the heavy wire terminals.The spools are then dipped into a pan of molten paraffin and boiled until the air bubbles cease to rise.The spools should be marked 1, 2, 10, 20, 30, and 50, according to the amount of wire each one contains as indicated in the table above.
The amateur experimenter will find many occasions when it is desirable to know the resistance of some of his electrical apparatus. Telephone receivers, telegraph relays, etc., are all graded according to their resistance in ohms. The measurement of resistance in any electrical instrument or circuit is usually accomplished by comparing its resistance with that of some known circuit, such as a coil of wire which has been previously tested.
The simplest method of measuring resistance is by means of a device known as the Wheatstone bridge. This instrument is very simple but at the same time is remarkably sensitive if properly made. A Wheatstone bridge is shown in Figure 116.
The base is a piece of well-seasoned hard wood, thirty inches long, six inches wide, and three-quarters of an inch thick.
Secure a long strip of No. 18 B. & S. gauge sheet-copper, one inch wide, and cut it into three pieces, making two of the pieces three inches long, and the other piece twenty-three and one-half inches long.
Mount the copper strips on the base, as shown, being very careful to make the distance between the inside edges of the end-pieces just twenty-five inches. The strips should be fastened to the base with small round-headed brass screws. Mount two binding-posts on each of the short strips in the positions shown in the illustration, and three on the long strip. These binding-posts should pass through the base and make firm contact with the strips.
Fig. 116.—Wheatstone Bridge.Fig. 116.—Wheatstone Bridge.
Fig. 116.—Wheatstone Bridge.
Then make a paper scale twenty-five inches long, and divide it into one hundred equal divisions one-quarter of an inch long. Mark every fifth division with a slightly longer line, and every tenth division with a double-length line.
Start at one end and number every ten divisions, then start at the other end and number them back, so that the scale reads 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, from right to left at the top and 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, from left to right at the bottom.
Solder a piece of No. 30 B. & S. gauge German-silver wire to one of the short copper strips opposite the end of the scale, and then stretch it tightly across the scale and solder it to the strip at the other end.
Make a knife-contact by flattening a piece of heavy copper wire as shown in Figure 117. Solder a piece of flexible wire, such as "lamp cord," at the other end. It is well to fit the contact with a small wooden handle, made by boring out a piece of dowel.
The instrument is now practically complete.
Fig. 117.—Knife-Contact.Fig. 117.—Knife-Contact.
Fig. 117.—Knife-Contact.
In order to use the Wheatstone bridge, it is necessary to have a set of resistances of known value. The resistance of any unknown circuit or piece of apparatus is found by comparing it with one of the known coils. It is just like going to a store and buying a pound of sugar. The grocer weighs out the sugar by balancing it on the scales with an iron weight of known value, and taking it for granted that the weight is correct, we would say that we have one, five, or ten pounds of sugar, as the case may be.
The Wheatstone bridge might be called a pair of "electrical scales" for weighing resistance by comparing an unknown coil with one which we know has a certain value.
The next step is to make up some standard resistance coils. Secure some No. 32 B. & S. gauge single-cotton-covered wire from an electrical dealer and cut into the following lengths, laying it straight on the floor but using care not to pull or stretch it.
1/2 ohm coil—3 feet 1/2 inch1 ohm coil—6 feet 1 1/4 inches2 ohm coil—12 feet 2 1/2 inches5 ohm coil—30 feet 6 1/4 inches10 ohm coil—61 feet20 ohm coil—122 feet30 ohm coil—183 feet50 ohm coil—305 feet
1/2 ohm coil—3 feet 1/2 inch
1 ohm coil—6 feet 1 1/4 inches
2 ohm coil—12 feet 2 1/2 inches
5 ohm coil—30 feet 6 1/4 inches
10 ohm coil—61 feet
20 ohm coil—122 feet
30 ohm coil—183 feet
50 ohm coil—305 feet
These lengths of wire are then wrapped on the spools in the following manner.
Fig. 118.—Resistance-Coil.Fig. 118.—Resistance-Coil.Ashows how the Wire is doubled and wound on the Spool.Bis the completed Coil.
Fig. 118.—Resistance-Coil.Ashows how the Wire is doubled and wound on the Spool.Bis the completed Coil.
This method of winding is known as the non-inductive method, because the windings do not generate a magnetic field, which might affect the galvanometer needle used in connection with the Wheatstone bridge as described later on.
Each length of wire should be doubled exactly in the middle, then wrapped on the spools like a single wire, the two ends being left free for soldering to the terminals as shown in Figure 118, B.
The spools may be the ordinary reels upon which cotton and sewing-silk are wrapped.
The terminals of the spools are pieces of stout copper wire, No. 12 or No. 14 B. & S. gauge. Two pieces of wire about three inches long are driven into holes bored in the ends of each spool. A small drop of solder is used permanently to secure the ends of the coil to each of the heavy wire terminals.
The spools are then dipped into a pan of molten paraffin and boiled until the air bubbles cease to rise.
The spools should be marked 1, 2, 10, 20, 30, and 50, according to the amount of wire each one contains as indicated in the table above.
How to Use a Wheatstone Bridge for Measuring ResistanceThe instrument is connected as in Figure 116.The unknown resistance or device to be measured is connected across the gap atB. One of the standard known coils is connected across the gap atA. A sensitive galvanometer or a telephone receiver and two cells of battery are also connected as shown.If a telephone receiver is used, place it to the ear. If a galvanometer is used instead, watch the needle carefully. Then move the sharp edge of the knife-contact over the scale along the German-silver "slide wire" until a point is reached when there is no deflection of the needle or no sound in the telephone receiver.If this point lies very far on one side or the other of the center division on the scale, substitute the next higher or lower known resistance spool until the point falls as near as possible to the center of the scale.When this point is found, note the reading on the scale carefully. Now comes the hardest part. Almost all my readers have no doubt progressed far enough in arithmetic to be able to carry on the following simple calculation in proportion which must be made in order to find out the resistance of the unknown coil.The unknown resistance, connected toB, bears the same ratio to the known coil, atA, that the number of divisions between the knife-contact and the right-hand end of the scale (lower row of figures) bears to the number of divisions between the knife-edge and the left-hand end of the scale (upper row of figures).We will suppose that a 5-ohm coil was used atAin a test, and the needle of the galvanometer stopped swinging when the knife-contact rested on the 60th division from the left-hand end, or on the 40th from the right. Then, in order to find the value of the unknown resistance atB, it is simply necessary to multiply the standard resistance atAby the number of left-hand divisions and divide the product by the number of right-hand divisions. The answer will be the resistance ofBin ohms.The calculation in this case would be as follows:5 X 40 = 200200/60 = 3.33 ohms3.33 ohms is the resistance ofB.This explanation may seem very long and complex, but if you will study it carefully you will find it to be very simple. When once you master it, you will be enabled to make many measurements of resistance which will add greatly to the interest and value of your experiments.BELLS, ALARMS, AND ANNUNCIATORS
The instrument is connected as in Figure 116.
The unknown resistance or device to be measured is connected across the gap atB. One of the standard known coils is connected across the gap atA. A sensitive galvanometer or a telephone receiver and two cells of battery are also connected as shown.
If a telephone receiver is used, place it to the ear. If a galvanometer is used instead, watch the needle carefully. Then move the sharp edge of the knife-contact over the scale along the German-silver "slide wire" until a point is reached when there is no deflection of the needle or no sound in the telephone receiver.
If this point lies very far on one side or the other of the center division on the scale, substitute the next higher or lower known resistance spool until the point falls as near as possible to the center of the scale.
When this point is found, note the reading on the scale carefully. Now comes the hardest part. Almost all my readers have no doubt progressed far enough in arithmetic to be able to carry on the following simple calculation in proportion which must be made in order to find out the resistance of the unknown coil.
The unknown resistance, connected toB, bears the same ratio to the known coil, atA, that the number of divisions between the knife-contact and the right-hand end of the scale (lower row of figures) bears to the number of divisions between the knife-edge and the left-hand end of the scale (upper row of figures).
We will suppose that a 5-ohm coil was used atAin a test, and the needle of the galvanometer stopped swinging when the knife-contact rested on the 60th division from the left-hand end, or on the 40th from the right. Then, in order to find the value of the unknown resistance atB, it is simply necessary to multiply the standard resistance atAby the number of left-hand divisions and divide the product by the number of right-hand divisions. The answer will be the resistance ofBin ohms.
The calculation in this case would be as follows:
5 X 40 = 200
5 X 40 = 200
200/60 = 3.33 ohms
200/60 = 3.33 ohms
3.33 ohms is the resistance ofB.
3.33 ohms is the resistance ofB.
This explanation may seem very long and complex, but if you will study it carefully you will find it to be very simple. When once you master it, you will be enabled to make many measurements of resistance which will add greatly to the interest and value of your experiments.
BELLS, ALARMS, AND ANNUNCIATORS