CHAPTER XII INDUCTION COILS

CHAPTER XII INDUCTION COILSA Medical Coil or shocking coil, as it is properly termed, is nothing more or less than a small induction coil, and consists of a core, a primary winding, a secondary winding, and an interrupter. The principle of an induction coil and that of magnetic induction have already been explained in Chapter V. It might be well for the readers to turn back to pages 89-91 and reread them.The human body possesses considerable resistance, and the voltage of one or two ordinary cells of battery is not sufficient to overcome that resistance and pass enough current through the body to be felt, unless under exceptional conditions.The simplest means employable for raising the voltage of a battery high enough to produce a shock is the medical coil.The first step in making such a coil is to roll up a paper tube, five-sixteenths of an inch in diameter inside, and two and one-half inches long. The outer end of the paper is carefully glued, so that it will not unroll. The tube is filled with pieces of iron wire two and one-half inches long which have been straightened by rolling between two boards. The size of the iron wire may vary from No. 20 to No. 24 B. & S. gauge. Enough should be slipped into the tube to pack it tightly and admit no more.A square block, 1 x 1 x 5-16 inches, is cut out of fiber or a close-grained hard wood and a hole three-eighths of an inch in diameter bored through the center. One end of the tube containing the core is smeared with glue and slipped into the block. The end of the tube is allowed to project through about one-sixteenth of an inch. A second block, in the form of a circle three-quarters of an inch in diameter, one-quarter of an inch thick, and having a three-eighths of an inch hole through the center, is glued on the opposite end.Fig. 154.—Details of Various Parts of a Medical Coil.Fig. 154.—Details of Various Parts of a Medical Coil.After the glue has dried, four small holes are drilled in the square head in the approximate positions shown by Figure 154. Four layers of No. 22 B. & S. gauge magnet wire (it may be either silk or cotton, double or single covered) is wound smoothly and carefully over the core. The terminals are led out of the holesaandb. The primary is covered with two or three layers of paper, and then enough secondary wound on to bring the total diameter of the coil to about eleven-sixteenths of an inch. The secondary wire must be much finer than the primary. It is possible to use any size from No. 32 to No. 36 B. & S. gauge and obtain good results. The insulation may be either single silk or single cotton.Fig. 155.—Details of Interrupter for Medical Coil.Fig. 155.—Details of Interrupter for Medical Coil.The secondary terminals are led out through the holescandd. It is perhaps a wise plan to re-enforce these leads with a heavier piece of wire, because otherwise they are easily broken.The interrupter is a simple arrangement capable of being made in several different ways. The drawing shows an arrangement which can be improved upon by any experimenters who are familiar with a medical coil. I have shown the simplest arrangement, so that all my readers will be able to build it, and those who want to improve it can do so.If a small piece of silver is soldered to the spring and to the contact-point it will give better results. The silver is easily secured by cutting up a ten-cent piece. One terminal of the primary is connected to the interrupter spring and the other to a binding-post. The contact-post is also connected to a binding-post. If a battery is connected to the two binding-posts, the current will flow from one post through the coil to the interrupter spring, through the spring to the contact post, and thence back to the battery, making a complete circuit. As soon as the current flows, however, it produces magnetism which draws the spring away from the contact and breaks the circuit, cutting off the magnetic pull. The spring flies back to the contact but is drawn forward again immediately and repeats the operation continuously at a high rate of speed.Fig. 156.—Completed Medical Coil.Fig. 156.—Completed Medical Coil.The secondary terminals are led out to two binding-posts to which are connected two electrodes or handles by means of flexible wires. The electrode may be made of two ordinary flat strips of sheet-metal or a piece of tubing. In the latter case, the wires may be connected by wedging them in with a cork. If the handles are grasped while the battery is connected to the primary posts and the interrupter is in operation a powerful shock will be felt. The shock may be regulated from a weak current that can hardly be felt to a very powerful one by providing the coil with a piece of iron tubing of about seven-eighths of an inch inner diameter and two inches long which will slip on and oh the coil. When the tube is all the way on, the shock is very mild, and when all the way off, the shock is very strong. Of course any intermediate strength may be secured at stages between the two extremes.The current from medical coils is often prescribed by physicians for rheumatism and nervous disorders, but must be properly applied. The coil just described is harmless. It will give a strong shock, but the only result is to make the person receiving it drop the handles and not be anxious to try it again.Spark-CoilsA "spark-coil" is one of the most interesting pieces of apparatus an experimenter can possess. The experiments that may be performed with its aid are varied and many.The purpose of a "spark-coil" is to generate enormously high voltages which are able to send sparks across an air space that ordinary currents of low voltage could not possibly pierce. The spark-coil is the same in principle as the small induction coils used as medical or shocking coils, but is made on a larger scale and is provided with a condenser connected across the terminals of the interrupter.Fig. 157.—Diagram showing Essential Parts of Induction Coil.Fig. 157.—Diagram showing Essential Parts of Induction Coil.It consists of a central iron core surrounded by a coil of heavy wire called the "primary," and by a second outside winding of wire known as the "secondary." The primary is connected to a few cells of battery in series with an interrupter. The interrupter makes and breaks the circuit, i. e., shuts the current on and off repeatedly.Every time that the current is "made" or broken, a high voltage is induced in the secondary. By means of the condenser connected across the interrupter terminals, the current at "make" is caused to take a considerable fraction of time to grow, while at "break," the cessation is instantaneous. The currents induced in the secondary at break are so powerful that they leap across the space in a brilliant torrent of sparks.Building a Spark-CoilPerhaps more attempts are made by experimenters to construct a spark-coil than any other piece of apparatus, and the results are usually poor. A spark-coil is not hard to construct, but it requires careful work and patience. It is not a job to be finished in a day, but time must be liberally expended in its construction. Satisfactory results are easily obtained by any one of ordinary mechanical ability if patience and care are used.Parts for spark-coils are for sale by many electrical houses, and it is possible to purchase a set of such machine-made parts for less than the separate materials usually cost.For the benefit of those who might wish to build a larger coil than the one described in the following text, a table showing the dimensions of two other sizes will be found.Fig. 158.—Empty Paper Tube, and Tube filled with Core Wire preparatory to winding on the Primary.Fig. 158.—Empty Paper Tube, and Tube filled with Core Wire preparatory to winding on the Primary.The coreis made of very soft iron wire about No. 20 or 22 B. & S. gauge, cut to exact length. Each piece should be six inches long. Iron wire may be purchased from electrical supply houses already cut to various lengths for twenty cents a pound. In view of the amount of labor required carefully to cut each piece to length and then straighten it out so that it will form a neat bundle, it is cheaper to purchase the wire already cut. Such wire has been annealed, i.e., softened by bringing to a red heat and then cooling slowly. In case the wire is purchased at a plumbing shop or a hardware store it must be annealed before it can be used. This is accomplished by tying the wire in a compact bundle and placing it in a wood fire where it will grow red-hot. When this stage is reached, cover the wire with ashes and allow the fire to die away.Fig. 159.—Illustrating the Various Steps in winding on the Primary and fastening the Ends of the Wire.Fig. 159.—Illustrating the Various Steps in winding on the Primary and fastening the Ends of the Wire.Cut a piece of tough wrapping paper into strips six inches long and about five inches wide. Wrap it around a stick or metal rod one-half of an inch in diameter, so as to form a tube six inches long and having a diameter of one-half of an inch. Glue the inside and outside edges of the paper so that the tube cannot unroll and then slip it off the stick.Fig. 160.—Complete Primary Winding and Core.Fig. 160.—Complete Primary Winding and Core.Fill the tube with the six-inch wires until it is packed tightly and no more can be slipped in.The primaryconsists of two layers of No. 18 B. & S. gauge cotton-covered wire wound over the core for a distance of five inches. One-half pound of wire is more than enough for one primary. The wire must be wound on very smoothly and carefully. In order to fasten the inside end so that it will not become loose, place a short piece of tape lengthwise of the core and wind on two or three turns over it. Then double the end back and complete the winding. After the first layer is finished, give it a coat of shellac and wind on the second layer. The end of the wire is wound with a piece of tape and fastened by slipping through a loop of tape embedded under the last few turns. The illustrations will explain more clearly just how this is accomplished. The second layer is then given a coat of shellac and allowed to dry. After it is dry, wrap about fifteen layers of paper which have been soaked in paraffin around the primary. This operation should be performed in a warm place, over a fire or lighted lamp where the paraffin may be kept soft, so that the paper will go on tightly.Fig. 161.—The Primary covered with Insulating Layer of Paper ready for the Secondary.Fig. 161.—The Primary covered with Insulating Layer of Paper ready for the Secondary.The coil is now ready to receive the secondary winding. The core and primary which have been described are suit-able for a secondary giving sparks from one-half to three-fourths of an inch long.The secondarywinding consists of several thousand turns of very fine wire wound on in smooth even layers with paper between each two layers.The following table shows the size and amount of wire required. In addition, about two pounds of paraffin and a pad of linen paper or typewriter paper will be required. The wire may be either enamel, cotton, or silk insulated. Single silk-covered wire is preferred.SIZE OF COILSIZE OF WIREAMOUNT1/2 inch36 B. & S.10 ounces1 inch34 B. & S.1 lb.1/2 inch34 B. & S.2 lbs.The means for supporting and turning the coil in order to wind on the secondary may be left somewhat to the ingenuity of the young experimenter. The following suggestion, however, is one which experience has proved to be well worth following out, and may be applied to other things than the construction of an induction coil. It seems to be the nature of most boys, for some reason or other, to be unwilling to spend time and labor on anything which will aid them in their work. They are always in such a hurry and so anxious to see something completed that they direct all their energy to that end rather than spend part of their time in constructing some little device which would really lighten the other work and go a long way towards insuring its successful completion.I have frequently given instructions for building an induction coil and placed particular stress upon winding the secondary, only to have such suggestions ignored in the anxious endeavor of boys to finish the coil as soon as possible. In every such instance the coil has been a failure.Fig. 162.—Simple Winding Device for winding the Secondary.Fig. 162.—Simple Winding Device for winding the Secondary.The illustration shows a simple form of winder, with which the operation of winding the secondary is a very slow one, but, on the other hand, it is possible to do very accurate, careful winding with the aid of such a device. The parts may all be made from wood.The chucks fit tightly over the ends of the core so that when the handle is turned, the coil will revolve also. The spring serves to keep the chucks snugly against the coil ends, so that they will not slip.From one-half to five-eighths of a pound of wire will be required to wind the coil. A large number of strips of thin paraffined or waxed paper must be cut five inches wide. The inside terminal, or "beginning" end of the wire is tied around the insulating tube near the left-hand end. The spool of wire must be placed in a position where it will revolve freely without strain on the wire. No. 36 is very fine and easily broken, so use the utmost care to guard against this mishap.Fig. 163.—Completed Secondary Winding.Fig. 163.—Completed Secondary Winding.Wind on a smooth, even layer of wire, permitting each turn to touch the other, but none to lap over. Carry the winding to within one-half inch of the ends of the insulating tube and then wind on two layers of the waxed paper.The paper must be put on smoothly and evenly, so as to afford a firm foundation for the next layer. The wire is wrapped around with the paper, so that the next layer starts one-half inch from the edge. A second layer is then wound on very carefully, stopping when it comes one-half inch from the edge. Two more layers of paper are put on, and the process repeated, alternately winding on paper and wire until the stated quantity of the latter has been used up. The layers of wire may occasionally be given a coating of shellac. This is a good insulator, and will serve to hold them together and prevent the wire from becoming loose.In winding the coil, remember that if at any point you allow the winding to become irregular or uneven, the irregularity will be much exaggerated on the succeeding layers. For this reason, do not allow any to occur. If the wire tends to go on unevenly, wrap an extra layer of thick paper around underneath so as to offer a smooth foundation, and you will find the difficulty remedied.Fig. 164.—Interrupter Parts.Fig. 164.—Interrupter Parts.An efficient vibrator for a coil cannot be easily made, and it is best to buy one which is already fitted with platinum points. The interrupter will play a very important part in the successful working of the coil, and its arrangement and construction are important. Interrupters like that shown in the illustration and used for automobile will be found the best.The condenser may be home-made. It consists of alternate sheets of tinfoil and paraffined paper, arranged in a pile as shown in the illustration. The following table gives the proper sizes for condensers for three different coils.SIZE OF SPARK-COILTINFOILNO. SHEETSSIZE OF SHEETS1/2 inch502 x 21 inch1007 x 51 1/2 inch1008 x 6The paper must be about one-half inch larger all the way around, so as to leave a good margin. The alternate sheets of tinfoil, that is, all on one side and all on the other, are connected.The condenser is connected directly across the terminals of the interrupter.Fig. 165.—Condenser.Fig. 165.—Condenser.There are various methods of mounting a coil, the most common being to place it in a box with the interrupter at one end. Perhaps, however, one of the neatest and also the simplest methods is to mount it in the manner shown in the illustration.The end-pieces are cut out of wood. No specific dimensions can be given, because the diameter of the coils will vary somewhat according to who winds them and how tightly they are made. The coil is enclosed in a tube made by rolling up a strip of cardboard and then giving it a coat of shellac. The tube may be covered by a strip of black cloth, so as to improve its appearance.Fig. 166.—Completed Coil.Fig. 166.—Completed Coil.The vibrator is mounted on the end. The core projects through a hole in the wood near the end of the vibrator spring so that the latter will be drawn in by the magnetism of the core when the current flows. The condenser may be placed in the hollow box which forms the base of the coil.The secondary terminals of the coil are mounted on a small strip of wood bridging the two coil ends.One terminal of the primary is connected to a binding-post mounted on the base, and the other led to the vibrator spring. The vibrator yoke is connected to a second binding-post on the base. One terminal of the condenser is connected to the spring, and the other to the yoke.Four cells of dry battery should be sufficient to run the coil and cause it to give a good one-half-inch spark if built according to the directions here given. The vibrator or interrupter will require adjusting and a position of the adjusting screw will soon be found where the coil works best.Experiments with a Spark-CoilElectrical Hands.Many extraordinary and interesting experiments may be performed with the aid of a spark-coil.The following experiment never fails to amuse a party of friends, and is mystifying and weird to the ordinary person, unacquainted with the secret of its operation.Figure 167 shows the arrangement of the apparatus. The primary of an ordinary one-inch spark induction coil is connected in series with a twelve-volt battery and telephone transmitter. A small switch is included in the circuit to break the current and prevent needless waste of the battery when the apparatus is not in immediate use. The secondary terminals of the induction coil are led by means of an insulated wire to the adjoining room where they terminate in a pair of scissors, or some other small metallic object which may be clasped in the hand.Each of two persons, wearing dry shoes or rubber-soled slippers, grasps the terminal of one wire in one hand. The other hand is placed flat against the ear of a third person, with a piece of dry linen paper intervening between the hands and the head. If a fourth person, in the room where the induction coil is located, then closes the small switch and speaks into the telephone transmitter, the person against whose ears the hands are being held will hear the speech very distinctly. The ticking of a watch held against the mouthpiece of the transmitter will be heard with startling clearness.Fig. 167.—Diagram showing how to connect the Apparatus for the "Electric Hands" Experiment.Fig. 167.—Diagram showing how to connect the Apparatus for the "Electric Hands" Experiment.The principle governing the operation of the apparatus is very simple. Almost every experimenter is familiar with the ordinary electrical condenser, which consists of alternate sheets of paraffined paper and tinfoil. When this is connected to a source of electricity of high potential, but not enough so as to puncture the paper dielectric, the alternate sheets of tinfoil will become oppositely charged and attract each other. If the circuit is then broken the sheets will lose their charge and also their attraction for one another. If the tinfoil sheets and paper are not pressed tightly together, there will be a slight movement of the tinfoil and paper which will correspond in frequency to any fluctuations of the charging current which may take place.The head of the third person and the hands held against his head act like three tinfoil sheets of a condenser, separated by two sheets of paper. The words spoken in the transmitter cause the current to fluctuate and the induction coil raises the potential of the current sufficiently to charge the condenser and cause a slight vibration of the paper dielectric. The vibrations correspond in strength and speed to those of the voice, and so the words spoken in the transmitter are audible to the person over whose ears the paper is pressed.Everything about the apparatus must be as dry as possible, to insure its successful operation. The people holding the wires in their hands should stand on a carpeted floor. Always be very careful to tighten the adjusting screw and block the interrupter on the coil, so that by no means can it possibly commence to operate, or the person listening, instead of "hearing things" will become the victim of a rather painful, practical joke.Geissler TubesThe most beautiful and surprising effects may be obtained by lighting Geissler tubes with a coil. The tubes are made in intricate and varied patterns of special glass, containing fluorescent minerals and salts, and are filled with different rarefied gases. When the tubes are connected to the secondary of a spark-coil by means of a wire fastened to the little rings at the end, and the coil is set in operation, they light up in the most wonderful way imaginable. The rarefied gases and minerals in the glass throw out beautiful iridescent colors, lighting up a dark room with a weird flickering light. Every tube is usually of a different pattern and has a combination of different colors. The most beautiful tubes are those provided with a double wall containing a fluorescent liquid, which heightens the color effects when the tube is lighted.Fig. 168.—Geissler Tubes.Fig. 168.—Geissler Tubes.Eight to ten tubes may be lighted at once on an ordinary coil by connecting them in series.Ghost LightIf you grasp the bulb of an old incandescent electric lamp in one hand and touch the base to one side of the secondary when the coil is in operation the bulb will emit a peculiar greenish light in the dark.Puncturing PaperIf you place a piece of heavy paper or cardboard between two sharp wires connected to the secondary of a spark-coil and start the coil working, the paper will be pierced.A Practical JokeThis action of the coil may be made the basis of an amusing joke. Offer a friend who may smoke cigarettes some cigarette paper which has been prepared in the following way.Fig. 169.—The Bulb will emit a Peculiar Greenish Light.Fig. 169.—The Bulb will emit a Peculiar Greenish Light.Place several sheets of the paper on a piece of sheet-metal which is connected to one side of the secondary. By means of an insulated handle so that you will not get a shock, move the other wire all over the surface of the cigarette paper. The paper will be pierced with numerous fine holes which are so fine that they can hardly be seen.If your friend uses any of the paper in making a cigarette and tries to light it he will waste a box of matches without being able to get one good puff, because the little invisible holes in the paper will spoil the draft. Perhaps he may quit smoking altogether.An Electric Garbage-canFig. 170.—An Electrified Garbage-can.Fig. 170.—An Electrified Garbage-can.If there are any dogs in your neighborhood that have a habit of extracting things from your ash-barrel or garbage-can, place the latter on a piece of dry wood. Lead a well insulated wire from one secondary terminal of your coil to the can. Ground the other secondary terminal. If you see a dog with his nose in the can press your key and start the coil working. It will not hurt the dog, but he will get the surprise of his life. He will go for home as fast as he can travel and will not touch that particular can again, even if it should contain some of the choicest canine delicacies.Photographing an Electric DischargeThe following experiment must be conducted in a dark room with the aid of a ruby photographic lamp, as otherwise the plates used would become lightstruck and spoiled.Fig. 171.—Jacob's Ladder.Fig. 171.—Jacob's Ladder.Placed an ordinary photographic plate on a piece of sheet-metal with the coated side of the plate upwards. Connect one of the secondary terminals of the spark-coil to the piece of sheet-metal.Then sift a thin film of dry starch powder, sulphur, or talcum through a piece of fine gauze on the plate. Lead a sharp-pointed wire from the other secondary terminal of the coil to the center of the plate and then push the key just long enough to make one spark.Wipe the powder off the plate and develop it in the usual manner of films and plates. If you cannot do developing yourself, place the plate back in its box and send it to some friend, or to a photographer.The result will be a negative showing a peculiar electric discharge, somewhat like sea-moss in appearance. No two such photographs will be alike and the greatest variety of new designs, etc., imaginable may be produced in this manner.Jacob’s LadderTake two pieces of bare copper wire about eight inches long and bend them at right angles. Place them in the secondary terminals of a spark-coil as in Figure 171. Bend them so that the vertical portions are about one-half of an inch apart at the bottom and one inch apart at the top. Start the coil working, and the sparks will run up the wires from the bottom to the top and appear very much like the rungs in a ladder.X-RaysMost young experimenters are unaware what a wonderful and interesting field is open to the possessor of a small X-ray tube.Small X-ray tubes which will operate satisfactorily on an inch and one-half spark-coil may be obtained from several electrical supply houses. They usually cost about four dollars and a half. With such a tube and afluoroscopeit is possible to see the bones in the human hand, the contents of a closed purse, etc.The tube is made of glass and contains a very high vacuum. The long end of the tube contains a platinum electrode called thecathode. The short end contains two electrodes calledanodes, one perpendicular to the tube and the other diagonal.The tube is usually clamped in a wooden holder called an X-ray tube stand. The tube should be so adjusted that the X-rays which are reflected from the diagonal anode will pass off in the direction shown by the dotted lines in Figure 174.The fluoroscope is a cone-shaped wooden box fitted with a screen composed of a sheet of paper covered with crystals of a chemical called platinum-barium-cyanide.Fig. 172.—An X-Ray Tube.Fig. 172.—An X-Ray Tube.The opposite end of the box is fitted with a covering of felt or velvet which shuts off the light around the eyes and nose when you look into the fluoroscope and hold it tightly against the face.A fluoroscope may be purchased complete, or the platinum-barium-cyanide screen purchased separately and mounted on a box as shown in Figure 173.The two anodes of the tube should be connected, and led to one terminal of a spark-coil capable of giving a spark at least one and one-half inches long. Another wire should be led from the cathode of the tube to the other terminal of the coil.Fig. 173.—Fluoroscope.Fig. 173.—Fluoroscope.When it is desired to inspect any object, such as the hand, it must be held close to the screen of the fluoroscope and placed between the latter and the tube in the path of the X-rays. The X-rays are thrown forth from the tube at an angle of 45 degrees from the diagonal anode.Look into the fluoroscope and it should appear to be filled with a green light. If not, the battery terminals connected to the primary of the coil should be reversed, so as to send the current through in the opposite direction.The X-rays will cause the chemicals on the screen to light up and give forth a peculiar green light. If the hand is held against the screen, between the screen and the tube, the X-rays will pass through the hand and cast a shadow on the screen. They do not pass through the bones as easily as they do through the flesh and so will cast a shadow of the bones in the hand on the screen, and if you look closely you will be able to see the various joints, etc. The interrupter on the coil should be carefully adjusted so that the light does not flicker too much.Fig. 174.—How to connect an X-Ray Tube to a Spark-Coil.Fig. 174.—How to connect an X-Ray Tube to a Spark-Coil.If it is desired to take X-ray pictures, a fluoroscope is unnecessary.Turn the tube around so that the X-rays point downward.Shut the battery current off so that the tube is not in operation until everything else is ready.Place an ordinary photographic plate, contained in an ordinary plate-holder, directly under the tube with the gelatin side of the plate upwards.Place the hand flat on the plate and lower the tube until it is only about three inches above the hand. Then start the coil working so that the tube lights up and permit it to run for about fifteen minutes without removing the hand. Then turn the current off and develop the plate in a dark room.It is possible to obtain a very good X-ray photograph of the hand in this manner. Photographs showing the skeleton of a mouse, nails in a board, coins in a purse, a bullet in a piece of wood, etc., are a few of the other objects which make interesting pictures.An X-Ray Photograph of the hand taken with the Outfit shown in Figure 174.An X-Ray Photograph of the hand taken with the Outfit shown in Figure 174. The arrows point to injuries to the bone of the third finger near the middle Joint Resulting in a Stiff Joint.TRANSFORMERS

A Medical Coil or shocking coil, as it is properly termed, is nothing more or less than a small induction coil, and consists of a core, a primary winding, a secondary winding, and an interrupter. The principle of an induction coil and that of magnetic induction have already been explained in Chapter V. It might be well for the readers to turn back to pages 89-91 and reread them.

The human body possesses considerable resistance, and the voltage of one or two ordinary cells of battery is not sufficient to overcome that resistance and pass enough current through the body to be felt, unless under exceptional conditions.

The simplest means employable for raising the voltage of a battery high enough to produce a shock is the medical coil.

The first step in making such a coil is to roll up a paper tube, five-sixteenths of an inch in diameter inside, and two and one-half inches long. The outer end of the paper is carefully glued, so that it will not unroll. The tube is filled with pieces of iron wire two and one-half inches long which have been straightened by rolling between two boards. The size of the iron wire may vary from No. 20 to No. 24 B. & S. gauge. Enough should be slipped into the tube to pack it tightly and admit no more.

A square block, 1 x 1 x 5-16 inches, is cut out of fiber or a close-grained hard wood and a hole three-eighths of an inch in diameter bored through the center. One end of the tube containing the core is smeared with glue and slipped into the block. The end of the tube is allowed to project through about one-sixteenth of an inch. A second block, in the form of a circle three-quarters of an inch in diameter, one-quarter of an inch thick, and having a three-eighths of an inch hole through the center, is glued on the opposite end.

Fig. 154.—Details of Various Parts of a Medical Coil.Fig. 154.—Details of Various Parts of a Medical Coil.

Fig. 154.—Details of Various Parts of a Medical Coil.

After the glue has dried, four small holes are drilled in the square head in the approximate positions shown by Figure 154. Four layers of No. 22 B. & S. gauge magnet wire (it may be either silk or cotton, double or single covered) is wound smoothly and carefully over the core. The terminals are led out of the holesaandb. The primary is covered with two or three layers of paper, and then enough secondary wound on to bring the total diameter of the coil to about eleven-sixteenths of an inch. The secondary wire must be much finer than the primary. It is possible to use any size from No. 32 to No. 36 B. & S. gauge and obtain good results. The insulation may be either single silk or single cotton.

Fig. 155.—Details of Interrupter for Medical Coil.Fig. 155.—Details of Interrupter for Medical Coil.

Fig. 155.—Details of Interrupter for Medical Coil.

The secondary terminals are led out through the holescandd. It is perhaps a wise plan to re-enforce these leads with a heavier piece of wire, because otherwise they are easily broken.

The interrupter is a simple arrangement capable of being made in several different ways. The drawing shows an arrangement which can be improved upon by any experimenters who are familiar with a medical coil. I have shown the simplest arrangement, so that all my readers will be able to build it, and those who want to improve it can do so.

If a small piece of silver is soldered to the spring and to the contact-point it will give better results. The silver is easily secured by cutting up a ten-cent piece. One terminal of the primary is connected to the interrupter spring and the other to a binding-post. The contact-post is also connected to a binding-post. If a battery is connected to the two binding-posts, the current will flow from one post through the coil to the interrupter spring, through the spring to the contact post, and thence back to the battery, making a complete circuit. As soon as the current flows, however, it produces magnetism which draws the spring away from the contact and breaks the circuit, cutting off the magnetic pull. The spring flies back to the contact but is drawn forward again immediately and repeats the operation continuously at a high rate of speed.

Fig. 156.—Completed Medical Coil.Fig. 156.—Completed Medical Coil.

Fig. 156.—Completed Medical Coil.

The secondary terminals are led out to two binding-posts to which are connected two electrodes or handles by means of flexible wires. The electrode may be made of two ordinary flat strips of sheet-metal or a piece of tubing. In the latter case, the wires may be connected by wedging them in with a cork. If the handles are grasped while the battery is connected to the primary posts and the interrupter is in operation a powerful shock will be felt. The shock may be regulated from a weak current that can hardly be felt to a very powerful one by providing the coil with a piece of iron tubing of about seven-eighths of an inch inner diameter and two inches long which will slip on and oh the coil. When the tube is all the way on, the shock is very mild, and when all the way off, the shock is very strong. Of course any intermediate strength may be secured at stages between the two extremes.

The current from medical coils is often prescribed by physicians for rheumatism and nervous disorders, but must be properly applied. The coil just described is harmless. It will give a strong shock, but the only result is to make the person receiving it drop the handles and not be anxious to try it again.

Spark-CoilsA "spark-coil" is one of the most interesting pieces of apparatus an experimenter can possess. The experiments that may be performed with its aid are varied and many.The purpose of a "spark-coil" is to generate enormously high voltages which are able to send sparks across an air space that ordinary currents of low voltage could not possibly pierce. The spark-coil is the same in principle as the small induction coils used as medical or shocking coils, but is made on a larger scale and is provided with a condenser connected across the terminals of the interrupter.Fig. 157.—Diagram showing Essential Parts of Induction Coil.Fig. 157.—Diagram showing Essential Parts of Induction Coil.It consists of a central iron core surrounded by a coil of heavy wire called the "primary," and by a second outside winding of wire known as the "secondary." The primary is connected to a few cells of battery in series with an interrupter. The interrupter makes and breaks the circuit, i. e., shuts the current on and off repeatedly.Every time that the current is "made" or broken, a high voltage is induced in the secondary. By means of the condenser connected across the interrupter terminals, the current at "make" is caused to take a considerable fraction of time to grow, while at "break," the cessation is instantaneous. The currents induced in the secondary at break are so powerful that they leap across the space in a brilliant torrent of sparks.

A "spark-coil" is one of the most interesting pieces of apparatus an experimenter can possess. The experiments that may be performed with its aid are varied and many.

The purpose of a "spark-coil" is to generate enormously high voltages which are able to send sparks across an air space that ordinary currents of low voltage could not possibly pierce. The spark-coil is the same in principle as the small induction coils used as medical or shocking coils, but is made on a larger scale and is provided with a condenser connected across the terminals of the interrupter.

Fig. 157.—Diagram showing Essential Parts of Induction Coil.Fig. 157.—Diagram showing Essential Parts of Induction Coil.

Fig. 157.—Diagram showing Essential Parts of Induction Coil.

It consists of a central iron core surrounded by a coil of heavy wire called the "primary," and by a second outside winding of wire known as the "secondary." The primary is connected to a few cells of battery in series with an interrupter. The interrupter makes and breaks the circuit, i. e., shuts the current on and off repeatedly.

Every time that the current is "made" or broken, a high voltage is induced in the secondary. By means of the condenser connected across the interrupter terminals, the current at "make" is caused to take a considerable fraction of time to grow, while at "break," the cessation is instantaneous. The currents induced in the secondary at break are so powerful that they leap across the space in a brilliant torrent of sparks.

Building a Spark-CoilPerhaps more attempts are made by experimenters to construct a spark-coil than any other piece of apparatus, and the results are usually poor. A spark-coil is not hard to construct, but it requires careful work and patience. It is not a job to be finished in a day, but time must be liberally expended in its construction. Satisfactory results are easily obtained by any one of ordinary mechanical ability if patience and care are used.Parts for spark-coils are for sale by many electrical houses, and it is possible to purchase a set of such machine-made parts for less than the separate materials usually cost.For the benefit of those who might wish to build a larger coil than the one described in the following text, a table showing the dimensions of two other sizes will be found.Fig. 158.—Empty Paper Tube, and Tube filled with Core Wire preparatory to winding on the Primary.Fig. 158.—Empty Paper Tube, and Tube filled with Core Wire preparatory to winding on the Primary.The coreis made of very soft iron wire about No. 20 or 22 B. & S. gauge, cut to exact length. Each piece should be six inches long. Iron wire may be purchased from electrical supply houses already cut to various lengths for twenty cents a pound. In view of the amount of labor required carefully to cut each piece to length and then straighten it out so that it will form a neat bundle, it is cheaper to purchase the wire already cut. Such wire has been annealed, i.e., softened by bringing to a red heat and then cooling slowly. In case the wire is purchased at a plumbing shop or a hardware store it must be annealed before it can be used. This is accomplished by tying the wire in a compact bundle and placing it in a wood fire where it will grow red-hot. When this stage is reached, cover the wire with ashes and allow the fire to die away.Fig. 159.—Illustrating the Various Steps in winding on the Primary and fastening the Ends of the Wire.Fig. 159.—Illustrating the Various Steps in winding on the Primary and fastening the Ends of the Wire.Cut a piece of tough wrapping paper into strips six inches long and about five inches wide. Wrap it around a stick or metal rod one-half of an inch in diameter, so as to form a tube six inches long and having a diameter of one-half of an inch. Glue the inside and outside edges of the paper so that the tube cannot unroll and then slip it off the stick.Fig. 160.—Complete Primary Winding and Core.Fig. 160.—Complete Primary Winding and Core.Fill the tube with the six-inch wires until it is packed tightly and no more can be slipped in.The primaryconsists of two layers of No. 18 B. & S. gauge cotton-covered wire wound over the core for a distance of five inches. One-half pound of wire is more than enough for one primary. The wire must be wound on very smoothly and carefully. In order to fasten the inside end so that it will not become loose, place a short piece of tape lengthwise of the core and wind on two or three turns over it. Then double the end back and complete the winding. After the first layer is finished, give it a coat of shellac and wind on the second layer. The end of the wire is wound with a piece of tape and fastened by slipping through a loop of tape embedded under the last few turns. The illustrations will explain more clearly just how this is accomplished. The second layer is then given a coat of shellac and allowed to dry. After it is dry, wrap about fifteen layers of paper which have been soaked in paraffin around the primary. This operation should be performed in a warm place, over a fire or lighted lamp where the paraffin may be kept soft, so that the paper will go on tightly.Fig. 161.—The Primary covered with Insulating Layer of Paper ready for the Secondary.Fig. 161.—The Primary covered with Insulating Layer of Paper ready for the Secondary.The coil is now ready to receive the secondary winding. The core and primary which have been described are suit-able for a secondary giving sparks from one-half to three-fourths of an inch long.The secondarywinding consists of several thousand turns of very fine wire wound on in smooth even layers with paper between each two layers.The following table shows the size and amount of wire required. In addition, about two pounds of paraffin and a pad of linen paper or typewriter paper will be required. The wire may be either enamel, cotton, or silk insulated. Single silk-covered wire is preferred.SIZE OF COILSIZE OF WIREAMOUNT1/2 inch36 B. & S.10 ounces1 inch34 B. & S.1 lb.1/2 inch34 B. & S.2 lbs.The means for supporting and turning the coil in order to wind on the secondary may be left somewhat to the ingenuity of the young experimenter. The following suggestion, however, is one which experience has proved to be well worth following out, and may be applied to other things than the construction of an induction coil. It seems to be the nature of most boys, for some reason or other, to be unwilling to spend time and labor on anything which will aid them in their work. They are always in such a hurry and so anxious to see something completed that they direct all their energy to that end rather than spend part of their time in constructing some little device which would really lighten the other work and go a long way towards insuring its successful completion.I have frequently given instructions for building an induction coil and placed particular stress upon winding the secondary, only to have such suggestions ignored in the anxious endeavor of boys to finish the coil as soon as possible. In every such instance the coil has been a failure.Fig. 162.—Simple Winding Device for winding the Secondary.Fig. 162.—Simple Winding Device for winding the Secondary.The illustration shows a simple form of winder, with which the operation of winding the secondary is a very slow one, but, on the other hand, it is possible to do very accurate, careful winding with the aid of such a device. The parts may all be made from wood.The chucks fit tightly over the ends of the core so that when the handle is turned, the coil will revolve also. The spring serves to keep the chucks snugly against the coil ends, so that they will not slip.From one-half to five-eighths of a pound of wire will be required to wind the coil. A large number of strips of thin paraffined or waxed paper must be cut five inches wide. The inside terminal, or "beginning" end of the wire is tied around the insulating tube near the left-hand end. The spool of wire must be placed in a position where it will revolve freely without strain on the wire. No. 36 is very fine and easily broken, so use the utmost care to guard against this mishap.Fig. 163.—Completed Secondary Winding.Fig. 163.—Completed Secondary Winding.Wind on a smooth, even layer of wire, permitting each turn to touch the other, but none to lap over. Carry the winding to within one-half inch of the ends of the insulating tube and then wind on two layers of the waxed paper.The paper must be put on smoothly and evenly, so as to afford a firm foundation for the next layer. The wire is wrapped around with the paper, so that the next layer starts one-half inch from the edge. A second layer is then wound on very carefully, stopping when it comes one-half inch from the edge. Two more layers of paper are put on, and the process repeated, alternately winding on paper and wire until the stated quantity of the latter has been used up. The layers of wire may occasionally be given a coating of shellac. This is a good insulator, and will serve to hold them together and prevent the wire from becoming loose.In winding the coil, remember that if at any point you allow the winding to become irregular or uneven, the irregularity will be much exaggerated on the succeeding layers. For this reason, do not allow any to occur. If the wire tends to go on unevenly, wrap an extra layer of thick paper around underneath so as to offer a smooth foundation, and you will find the difficulty remedied.Fig. 164.—Interrupter Parts.Fig. 164.—Interrupter Parts.An efficient vibrator for a coil cannot be easily made, and it is best to buy one which is already fitted with platinum points. The interrupter will play a very important part in the successful working of the coil, and its arrangement and construction are important. Interrupters like that shown in the illustration and used for automobile will be found the best.The condenser may be home-made. It consists of alternate sheets of tinfoil and paraffined paper, arranged in a pile as shown in the illustration. The following table gives the proper sizes for condensers for three different coils.SIZE OF SPARK-COILTINFOILNO. SHEETSSIZE OF SHEETS1/2 inch502 x 21 inch1007 x 51 1/2 inch1008 x 6The paper must be about one-half inch larger all the way around, so as to leave a good margin. The alternate sheets of tinfoil, that is, all on one side and all on the other, are connected.The condenser is connected directly across the terminals of the interrupter.Fig. 165.—Condenser.Fig. 165.—Condenser.There are various methods of mounting a coil, the most common being to place it in a box with the interrupter at one end. Perhaps, however, one of the neatest and also the simplest methods is to mount it in the manner shown in the illustration.The end-pieces are cut out of wood. No specific dimensions can be given, because the diameter of the coils will vary somewhat according to who winds them and how tightly they are made. The coil is enclosed in a tube made by rolling up a strip of cardboard and then giving it a coat of shellac. The tube may be covered by a strip of black cloth, so as to improve its appearance.Fig. 166.—Completed Coil.Fig. 166.—Completed Coil.The vibrator is mounted on the end. The core projects through a hole in the wood near the end of the vibrator spring so that the latter will be drawn in by the magnetism of the core when the current flows. The condenser may be placed in the hollow box which forms the base of the coil.The secondary terminals of the coil are mounted on a small strip of wood bridging the two coil ends.One terminal of the primary is connected to a binding-post mounted on the base, and the other led to the vibrator spring. The vibrator yoke is connected to a second binding-post on the base. One terminal of the condenser is connected to the spring, and the other to the yoke.Four cells of dry battery should be sufficient to run the coil and cause it to give a good one-half-inch spark if built according to the directions here given. The vibrator or interrupter will require adjusting and a position of the adjusting screw will soon be found where the coil works best.

Perhaps more attempts are made by experimenters to construct a spark-coil than any other piece of apparatus, and the results are usually poor. A spark-coil is not hard to construct, but it requires careful work and patience. It is not a job to be finished in a day, but time must be liberally expended in its construction. Satisfactory results are easily obtained by any one of ordinary mechanical ability if patience and care are used.

Parts for spark-coils are for sale by many electrical houses, and it is possible to purchase a set of such machine-made parts for less than the separate materials usually cost.

For the benefit of those who might wish to build a larger coil than the one described in the following text, a table showing the dimensions of two other sizes will be found.

Fig. 158.—Empty Paper Tube, and Tube filled with Core Wire preparatory to winding on the Primary.Fig. 158.—Empty Paper Tube, and Tube filled with Core Wire preparatory to winding on the Primary.

Fig. 158.—Empty Paper Tube, and Tube filled with Core Wire preparatory to winding on the Primary.

The coreis made of very soft iron wire about No. 20 or 22 B. & S. gauge, cut to exact length. Each piece should be six inches long. Iron wire may be purchased from electrical supply houses already cut to various lengths for twenty cents a pound. In view of the amount of labor required carefully to cut each piece to length and then straighten it out so that it will form a neat bundle, it is cheaper to purchase the wire already cut. Such wire has been annealed, i.e., softened by bringing to a red heat and then cooling slowly. In case the wire is purchased at a plumbing shop or a hardware store it must be annealed before it can be used. This is accomplished by tying the wire in a compact bundle and placing it in a wood fire where it will grow red-hot. When this stage is reached, cover the wire with ashes and allow the fire to die away.

Fig. 159.—Illustrating the Various Steps in winding on the Primary and fastening the Ends of the Wire.Fig. 159.—Illustrating the Various Steps in winding on the Primary and fastening the Ends of the Wire.

Fig. 159.—Illustrating the Various Steps in winding on the Primary and fastening the Ends of the Wire.

Cut a piece of tough wrapping paper into strips six inches long and about five inches wide. Wrap it around a stick or metal rod one-half of an inch in diameter, so as to form a tube six inches long and having a diameter of one-half of an inch. Glue the inside and outside edges of the paper so that the tube cannot unroll and then slip it off the stick.

Fig. 160.—Complete Primary Winding and Core.Fig. 160.—Complete Primary Winding and Core.

Fig. 160.—Complete Primary Winding and Core.

Fill the tube with the six-inch wires until it is packed tightly and no more can be slipped in.

The primaryconsists of two layers of No. 18 B. & S. gauge cotton-covered wire wound over the core for a distance of five inches. One-half pound of wire is more than enough for one primary. The wire must be wound on very smoothly and carefully. In order to fasten the inside end so that it will not become loose, place a short piece of tape lengthwise of the core and wind on two or three turns over it. Then double the end back and complete the winding. After the first layer is finished, give it a coat of shellac and wind on the second layer. The end of the wire is wound with a piece of tape and fastened by slipping through a loop of tape embedded under the last few turns. The illustrations will explain more clearly just how this is accomplished. The second layer is then given a coat of shellac and allowed to dry. After it is dry, wrap about fifteen layers of paper which have been soaked in paraffin around the primary. This operation should be performed in a warm place, over a fire or lighted lamp where the paraffin may be kept soft, so that the paper will go on tightly.

Fig. 161.—The Primary covered with Insulating Layer of Paper ready for the Secondary.Fig. 161.—The Primary covered with Insulating Layer of Paper ready for the Secondary.

Fig. 161.—The Primary covered with Insulating Layer of Paper ready for the Secondary.

The coil is now ready to receive the secondary winding. The core and primary which have been described are suit-able for a secondary giving sparks from one-half to three-fourths of an inch long.

The secondarywinding consists of several thousand turns of very fine wire wound on in smooth even layers with paper between each two layers.

The following table shows the size and amount of wire required. In addition, about two pounds of paraffin and a pad of linen paper or typewriter paper will be required. The wire may be either enamel, cotton, or silk insulated. Single silk-covered wire is preferred.

SIZE OF COIL

SIZE OF WIRE

AMOUNT

1/2 inch

36 B. & S.

10 ounces

1 inch

34 B. & S.

1 lb.

1/2 inch

34 B. & S.

2 lbs.

The means for supporting and turning the coil in order to wind on the secondary may be left somewhat to the ingenuity of the young experimenter. The following suggestion, however, is one which experience has proved to be well worth following out, and may be applied to other things than the construction of an induction coil. It seems to be the nature of most boys, for some reason or other, to be unwilling to spend time and labor on anything which will aid them in their work. They are always in such a hurry and so anxious to see something completed that they direct all their energy to that end rather than spend part of their time in constructing some little device which would really lighten the other work and go a long way towards insuring its successful completion.

I have frequently given instructions for building an induction coil and placed particular stress upon winding the secondary, only to have such suggestions ignored in the anxious endeavor of boys to finish the coil as soon as possible. In every such instance the coil has been a failure.

Fig. 162.—Simple Winding Device for winding the Secondary.Fig. 162.—Simple Winding Device for winding the Secondary.

Fig. 162.—Simple Winding Device for winding the Secondary.

The illustration shows a simple form of winder, with which the operation of winding the secondary is a very slow one, but, on the other hand, it is possible to do very accurate, careful winding with the aid of such a device. The parts may all be made from wood.

The chucks fit tightly over the ends of the core so that when the handle is turned, the coil will revolve also. The spring serves to keep the chucks snugly against the coil ends, so that they will not slip.

From one-half to five-eighths of a pound of wire will be required to wind the coil. A large number of strips of thin paraffined or waxed paper must be cut five inches wide. The inside terminal, or "beginning" end of the wire is tied around the insulating tube near the left-hand end. The spool of wire must be placed in a position where it will revolve freely without strain on the wire. No. 36 is very fine and easily broken, so use the utmost care to guard against this mishap.

Fig. 163.—Completed Secondary Winding.Fig. 163.—Completed Secondary Winding.

Fig. 163.—Completed Secondary Winding.

Wind on a smooth, even layer of wire, permitting each turn to touch the other, but none to lap over. Carry the winding to within one-half inch of the ends of the insulating tube and then wind on two layers of the waxed paper.

The paper must be put on smoothly and evenly, so as to afford a firm foundation for the next layer. The wire is wrapped around with the paper, so that the next layer starts one-half inch from the edge. A second layer is then wound on very carefully, stopping when it comes one-half inch from the edge. Two more layers of paper are put on, and the process repeated, alternately winding on paper and wire until the stated quantity of the latter has been used up. The layers of wire may occasionally be given a coating of shellac. This is a good insulator, and will serve to hold them together and prevent the wire from becoming loose.

In winding the coil, remember that if at any point you allow the winding to become irregular or uneven, the irregularity will be much exaggerated on the succeeding layers. For this reason, do not allow any to occur. If the wire tends to go on unevenly, wrap an extra layer of thick paper around underneath so as to offer a smooth foundation, and you will find the difficulty remedied.

Fig. 164.—Interrupter Parts.Fig. 164.—Interrupter Parts.

Fig. 164.—Interrupter Parts.

An efficient vibrator for a coil cannot be easily made, and it is best to buy one which is already fitted with platinum points. The interrupter will play a very important part in the successful working of the coil, and its arrangement and construction are important. Interrupters like that shown in the illustration and used for automobile will be found the best.

The condenser may be home-made. It consists of alternate sheets of tinfoil and paraffined paper, arranged in a pile as shown in the illustration. The following table gives the proper sizes for condensers for three different coils.

SIZE OF SPARK-COIL

TINFOIL

NO. SHEETS

SIZE OF SHEETS

1/2 inch

2 x 2

1 inch

100

7 x 5

1 1/2 inch

100

8 x 6

The paper must be about one-half inch larger all the way around, so as to leave a good margin. The alternate sheets of tinfoil, that is, all on one side and all on the other, are connected.

The condenser is connected directly across the terminals of the interrupter.

Fig. 165.—Condenser.Fig. 165.—Condenser.

Fig. 165.—Condenser.

There are various methods of mounting a coil, the most common being to place it in a box with the interrupter at one end. Perhaps, however, one of the neatest and also the simplest methods is to mount it in the manner shown in the illustration.

The end-pieces are cut out of wood. No specific dimensions can be given, because the diameter of the coils will vary somewhat according to who winds them and how tightly they are made. The coil is enclosed in a tube made by rolling up a strip of cardboard and then giving it a coat of shellac. The tube may be covered by a strip of black cloth, so as to improve its appearance.

Fig. 166.—Completed Coil.Fig. 166.—Completed Coil.

Fig. 166.—Completed Coil.

The vibrator is mounted on the end. The core projects through a hole in the wood near the end of the vibrator spring so that the latter will be drawn in by the magnetism of the core when the current flows. The condenser may be placed in the hollow box which forms the base of the coil.

The secondary terminals of the coil are mounted on a small strip of wood bridging the two coil ends.

One terminal of the primary is connected to a binding-post mounted on the base, and the other led to the vibrator spring. The vibrator yoke is connected to a second binding-post on the base. One terminal of the condenser is connected to the spring, and the other to the yoke.

Four cells of dry battery should be sufficient to run the coil and cause it to give a good one-half-inch spark if built according to the directions here given. The vibrator or interrupter will require adjusting and a position of the adjusting screw will soon be found where the coil works best.

Experiments with a Spark-CoilElectrical Hands.Many extraordinary and interesting experiments may be performed with the aid of a spark-coil.The following experiment never fails to amuse a party of friends, and is mystifying and weird to the ordinary person, unacquainted with the secret of its operation.Figure 167 shows the arrangement of the apparatus. The primary of an ordinary one-inch spark induction coil is connected in series with a twelve-volt battery and telephone transmitter. A small switch is included in the circuit to break the current and prevent needless waste of the battery when the apparatus is not in immediate use. The secondary terminals of the induction coil are led by means of an insulated wire to the adjoining room where they terminate in a pair of scissors, or some other small metallic object which may be clasped in the hand.Each of two persons, wearing dry shoes or rubber-soled slippers, grasps the terminal of one wire in one hand. The other hand is placed flat against the ear of a third person, with a piece of dry linen paper intervening between the hands and the head. If a fourth person, in the room where the induction coil is located, then closes the small switch and speaks into the telephone transmitter, the person against whose ears the hands are being held will hear the speech very distinctly. The ticking of a watch held against the mouthpiece of the transmitter will be heard with startling clearness.Fig. 167.—Diagram showing how to connect the Apparatus for the "Electric Hands" Experiment.Fig. 167.—Diagram showing how to connect the Apparatus for the "Electric Hands" Experiment.The principle governing the operation of the apparatus is very simple. Almost every experimenter is familiar with the ordinary electrical condenser, which consists of alternate sheets of paraffined paper and tinfoil. When this is connected to a source of electricity of high potential, but not enough so as to puncture the paper dielectric, the alternate sheets of tinfoil will become oppositely charged and attract each other. If the circuit is then broken the sheets will lose their charge and also their attraction for one another. If the tinfoil sheets and paper are not pressed tightly together, there will be a slight movement of the tinfoil and paper which will correspond in frequency to any fluctuations of the charging current which may take place.The head of the third person and the hands held against his head act like three tinfoil sheets of a condenser, separated by two sheets of paper. The words spoken in the transmitter cause the current to fluctuate and the induction coil raises the potential of the current sufficiently to charge the condenser and cause a slight vibration of the paper dielectric. The vibrations correspond in strength and speed to those of the voice, and so the words spoken in the transmitter are audible to the person over whose ears the paper is pressed.Everything about the apparatus must be as dry as possible, to insure its successful operation. The people holding the wires in their hands should stand on a carpeted floor. Always be very careful to tighten the adjusting screw and block the interrupter on the coil, so that by no means can it possibly commence to operate, or the person listening, instead of "hearing things" will become the victim of a rather painful, practical joke.

Electrical Hands.Many extraordinary and interesting experiments may be performed with the aid of a spark-coil.

The following experiment never fails to amuse a party of friends, and is mystifying and weird to the ordinary person, unacquainted with the secret of its operation.

Figure 167 shows the arrangement of the apparatus. The primary of an ordinary one-inch spark induction coil is connected in series with a twelve-volt battery and telephone transmitter. A small switch is included in the circuit to break the current and prevent needless waste of the battery when the apparatus is not in immediate use. The secondary terminals of the induction coil are led by means of an insulated wire to the adjoining room where they terminate in a pair of scissors, or some other small metallic object which may be clasped in the hand.

Each of two persons, wearing dry shoes or rubber-soled slippers, grasps the terminal of one wire in one hand. The other hand is placed flat against the ear of a third person, with a piece of dry linen paper intervening between the hands and the head. If a fourth person, in the room where the induction coil is located, then closes the small switch and speaks into the telephone transmitter, the person against whose ears the hands are being held will hear the speech very distinctly. The ticking of a watch held against the mouthpiece of the transmitter will be heard with startling clearness.

Fig. 167.—Diagram showing how to connect the Apparatus for the "Electric Hands" Experiment.Fig. 167.—Diagram showing how to connect the Apparatus for the "Electric Hands" Experiment.

Fig. 167.—Diagram showing how to connect the Apparatus for the "Electric Hands" Experiment.

The principle governing the operation of the apparatus is very simple. Almost every experimenter is familiar with the ordinary electrical condenser, which consists of alternate sheets of paraffined paper and tinfoil. When this is connected to a source of electricity of high potential, but not enough so as to puncture the paper dielectric, the alternate sheets of tinfoil will become oppositely charged and attract each other. If the circuit is then broken the sheets will lose their charge and also their attraction for one another. If the tinfoil sheets and paper are not pressed tightly together, there will be a slight movement of the tinfoil and paper which will correspond in frequency to any fluctuations of the charging current which may take place.

The head of the third person and the hands held against his head act like three tinfoil sheets of a condenser, separated by two sheets of paper. The words spoken in the transmitter cause the current to fluctuate and the induction coil raises the potential of the current sufficiently to charge the condenser and cause a slight vibration of the paper dielectric. The vibrations correspond in strength and speed to those of the voice, and so the words spoken in the transmitter are audible to the person over whose ears the paper is pressed.

Everything about the apparatus must be as dry as possible, to insure its successful operation. The people holding the wires in their hands should stand on a carpeted floor. Always be very careful to tighten the adjusting screw and block the interrupter on the coil, so that by no means can it possibly commence to operate, or the person listening, instead of "hearing things" will become the victim of a rather painful, practical joke.

Geissler TubesThe most beautiful and surprising effects may be obtained by lighting Geissler tubes with a coil. The tubes are made in intricate and varied patterns of special glass, containing fluorescent minerals and salts, and are filled with different rarefied gases. When the tubes are connected to the secondary of a spark-coil by means of a wire fastened to the little rings at the end, and the coil is set in operation, they light up in the most wonderful way imaginable. The rarefied gases and minerals in the glass throw out beautiful iridescent colors, lighting up a dark room with a weird flickering light. Every tube is usually of a different pattern and has a combination of different colors. The most beautiful tubes are those provided with a double wall containing a fluorescent liquid, which heightens the color effects when the tube is lighted.Fig. 168.—Geissler Tubes.Fig. 168.—Geissler Tubes.Eight to ten tubes may be lighted at once on an ordinary coil by connecting them in series.

The most beautiful and surprising effects may be obtained by lighting Geissler tubes with a coil. The tubes are made in intricate and varied patterns of special glass, containing fluorescent minerals and salts, and are filled with different rarefied gases. When the tubes are connected to the secondary of a spark-coil by means of a wire fastened to the little rings at the end, and the coil is set in operation, they light up in the most wonderful way imaginable. The rarefied gases and minerals in the glass throw out beautiful iridescent colors, lighting up a dark room with a weird flickering light. Every tube is usually of a different pattern and has a combination of different colors. The most beautiful tubes are those provided with a double wall containing a fluorescent liquid, which heightens the color effects when the tube is lighted.

Fig. 168.—Geissler Tubes.Fig. 168.—Geissler Tubes.

Fig. 168.—Geissler Tubes.

Eight to ten tubes may be lighted at once on an ordinary coil by connecting them in series.

Ghost LightIf you grasp the bulb of an old incandescent electric lamp in one hand and touch the base to one side of the secondary when the coil is in operation the bulb will emit a peculiar greenish light in the dark.

If you grasp the bulb of an old incandescent electric lamp in one hand and touch the base to one side of the secondary when the coil is in operation the bulb will emit a peculiar greenish light in the dark.

Puncturing PaperIf you place a piece of heavy paper or cardboard between two sharp wires connected to the secondary of a spark-coil and start the coil working, the paper will be pierced.

If you place a piece of heavy paper or cardboard between two sharp wires connected to the secondary of a spark-coil and start the coil working, the paper will be pierced.

A Practical JokeThis action of the coil may be made the basis of an amusing joke. Offer a friend who may smoke cigarettes some cigarette paper which has been prepared in the following way.Fig. 169.—The Bulb will emit a Peculiar Greenish Light.Fig. 169.—The Bulb will emit a Peculiar Greenish Light.Place several sheets of the paper on a piece of sheet-metal which is connected to one side of the secondary. By means of an insulated handle so that you will not get a shock, move the other wire all over the surface of the cigarette paper. The paper will be pierced with numerous fine holes which are so fine that they can hardly be seen.If your friend uses any of the paper in making a cigarette and tries to light it he will waste a box of matches without being able to get one good puff, because the little invisible holes in the paper will spoil the draft. Perhaps he may quit smoking altogether.

This action of the coil may be made the basis of an amusing joke. Offer a friend who may smoke cigarettes some cigarette paper which has been prepared in the following way.

Fig. 169.—The Bulb will emit a Peculiar Greenish Light.Fig. 169.—The Bulb will emit a Peculiar Greenish Light.

Fig. 169.—The Bulb will emit a Peculiar Greenish Light.

Place several sheets of the paper on a piece of sheet-metal which is connected to one side of the secondary. By means of an insulated handle so that you will not get a shock, move the other wire all over the surface of the cigarette paper. The paper will be pierced with numerous fine holes which are so fine that they can hardly be seen.

If your friend uses any of the paper in making a cigarette and tries to light it he will waste a box of matches without being able to get one good puff, because the little invisible holes in the paper will spoil the draft. Perhaps he may quit smoking altogether.

An Electric Garbage-canFig. 170.—An Electrified Garbage-can.Fig. 170.—An Electrified Garbage-can.If there are any dogs in your neighborhood that have a habit of extracting things from your ash-barrel or garbage-can, place the latter on a piece of dry wood. Lead a well insulated wire from one secondary terminal of your coil to the can. Ground the other secondary terminal. If you see a dog with his nose in the can press your key and start the coil working. It will not hurt the dog, but he will get the surprise of his life. He will go for home as fast as he can travel and will not touch that particular can again, even if it should contain some of the choicest canine delicacies.

Fig. 170.—An Electrified Garbage-can.Fig. 170.—An Electrified Garbage-can.

Fig. 170.—An Electrified Garbage-can.

If there are any dogs in your neighborhood that have a habit of extracting things from your ash-barrel or garbage-can, place the latter on a piece of dry wood. Lead a well insulated wire from one secondary terminal of your coil to the can. Ground the other secondary terminal. If you see a dog with his nose in the can press your key and start the coil working. It will not hurt the dog, but he will get the surprise of his life. He will go for home as fast as he can travel and will not touch that particular can again, even if it should contain some of the choicest canine delicacies.

Photographing an Electric DischargeThe following experiment must be conducted in a dark room with the aid of a ruby photographic lamp, as otherwise the plates used would become lightstruck and spoiled.Fig. 171.—Jacob's Ladder.Fig. 171.—Jacob's Ladder.Placed an ordinary photographic plate on a piece of sheet-metal with the coated side of the plate upwards. Connect one of the secondary terminals of the spark-coil to the piece of sheet-metal.Then sift a thin film of dry starch powder, sulphur, or talcum through a piece of fine gauze on the plate. Lead a sharp-pointed wire from the other secondary terminal of the coil to the center of the plate and then push the key just long enough to make one spark.Wipe the powder off the plate and develop it in the usual manner of films and plates. If you cannot do developing yourself, place the plate back in its box and send it to some friend, or to a photographer.The result will be a negative showing a peculiar electric discharge, somewhat like sea-moss in appearance. No two such photographs will be alike and the greatest variety of new designs, etc., imaginable may be produced in this manner.

The following experiment must be conducted in a dark room with the aid of a ruby photographic lamp, as otherwise the plates used would become lightstruck and spoiled.

Fig. 171.—Jacob's Ladder.Fig. 171.—Jacob's Ladder.

Fig. 171.—Jacob's Ladder.

Placed an ordinary photographic plate on a piece of sheet-metal with the coated side of the plate upwards. Connect one of the secondary terminals of the spark-coil to the piece of sheet-metal.

Then sift a thin film of dry starch powder, sulphur, or talcum through a piece of fine gauze on the plate. Lead a sharp-pointed wire from the other secondary terminal of the coil to the center of the plate and then push the key just long enough to make one spark.

Wipe the powder off the plate and develop it in the usual manner of films and plates. If you cannot do developing yourself, place the plate back in its box and send it to some friend, or to a photographer.

The result will be a negative showing a peculiar electric discharge, somewhat like sea-moss in appearance. No two such photographs will be alike and the greatest variety of new designs, etc., imaginable may be produced in this manner.

Jacob’s LadderTake two pieces of bare copper wire about eight inches long and bend them at right angles. Place them in the secondary terminals of a spark-coil as in Figure 171. Bend them so that the vertical portions are about one-half of an inch apart at the bottom and one inch apart at the top. Start the coil working, and the sparks will run up the wires from the bottom to the top and appear very much like the rungs in a ladder.

Take two pieces of bare copper wire about eight inches long and bend them at right angles. Place them in the secondary terminals of a spark-coil as in Figure 171. Bend them so that the vertical portions are about one-half of an inch apart at the bottom and one inch apart at the top. Start the coil working, and the sparks will run up the wires from the bottom to the top and appear very much like the rungs in a ladder.

X-RaysMost young experimenters are unaware what a wonderful and interesting field is open to the possessor of a small X-ray tube.Small X-ray tubes which will operate satisfactorily on an inch and one-half spark-coil may be obtained from several electrical supply houses. They usually cost about four dollars and a half. With such a tube and afluoroscopeit is possible to see the bones in the human hand, the contents of a closed purse, etc.The tube is made of glass and contains a very high vacuum. The long end of the tube contains a platinum electrode called thecathode. The short end contains two electrodes calledanodes, one perpendicular to the tube and the other diagonal.The tube is usually clamped in a wooden holder called an X-ray tube stand. The tube should be so adjusted that the X-rays which are reflected from the diagonal anode will pass off in the direction shown by the dotted lines in Figure 174.The fluoroscope is a cone-shaped wooden box fitted with a screen composed of a sheet of paper covered with crystals of a chemical called platinum-barium-cyanide.Fig. 172.—An X-Ray Tube.Fig. 172.—An X-Ray Tube.The opposite end of the box is fitted with a covering of felt or velvet which shuts off the light around the eyes and nose when you look into the fluoroscope and hold it tightly against the face.A fluoroscope may be purchased complete, or the platinum-barium-cyanide screen purchased separately and mounted on a box as shown in Figure 173.The two anodes of the tube should be connected, and led to one terminal of a spark-coil capable of giving a spark at least one and one-half inches long. Another wire should be led from the cathode of the tube to the other terminal of the coil.Fig. 173.—Fluoroscope.Fig. 173.—Fluoroscope.When it is desired to inspect any object, such as the hand, it must be held close to the screen of the fluoroscope and placed between the latter and the tube in the path of the X-rays. The X-rays are thrown forth from the tube at an angle of 45 degrees from the diagonal anode.Look into the fluoroscope and it should appear to be filled with a green light. If not, the battery terminals connected to the primary of the coil should be reversed, so as to send the current through in the opposite direction.The X-rays will cause the chemicals on the screen to light up and give forth a peculiar green light. If the hand is held against the screen, between the screen and the tube, the X-rays will pass through the hand and cast a shadow on the screen. They do not pass through the bones as easily as they do through the flesh and so will cast a shadow of the bones in the hand on the screen, and if you look closely you will be able to see the various joints, etc. The interrupter on the coil should be carefully adjusted so that the light does not flicker too much.Fig. 174.—How to connect an X-Ray Tube to a Spark-Coil.Fig. 174.—How to connect an X-Ray Tube to a Spark-Coil.If it is desired to take X-ray pictures, a fluoroscope is unnecessary.Turn the tube around so that the X-rays point downward.Shut the battery current off so that the tube is not in operation until everything else is ready.Place an ordinary photographic plate, contained in an ordinary plate-holder, directly under the tube with the gelatin side of the plate upwards.Place the hand flat on the plate and lower the tube until it is only about three inches above the hand. Then start the coil working so that the tube lights up and permit it to run for about fifteen minutes without removing the hand. Then turn the current off and develop the plate in a dark room.It is possible to obtain a very good X-ray photograph of the hand in this manner. Photographs showing the skeleton of a mouse, nails in a board, coins in a purse, a bullet in a piece of wood, etc., are a few of the other objects which make interesting pictures.An X-Ray Photograph of the hand taken with the Outfit shown in Figure 174.An X-Ray Photograph of the hand taken with the Outfit shown in Figure 174. The arrows point to injuries to the bone of the third finger near the middle Joint Resulting in a Stiff Joint.TRANSFORMERS

Most young experimenters are unaware what a wonderful and interesting field is open to the possessor of a small X-ray tube.

Small X-ray tubes which will operate satisfactorily on an inch and one-half spark-coil may be obtained from several electrical supply houses. They usually cost about four dollars and a half. With such a tube and afluoroscopeit is possible to see the bones in the human hand, the contents of a closed purse, etc.

The tube is made of glass and contains a very high vacuum. The long end of the tube contains a platinum electrode called thecathode. The short end contains two electrodes calledanodes, one perpendicular to the tube and the other diagonal.

The tube is usually clamped in a wooden holder called an X-ray tube stand. The tube should be so adjusted that the X-rays which are reflected from the diagonal anode will pass off in the direction shown by the dotted lines in Figure 174.

The fluoroscope is a cone-shaped wooden box fitted with a screen composed of a sheet of paper covered with crystals of a chemical called platinum-barium-cyanide.

Fig. 172.—An X-Ray Tube.Fig. 172.—An X-Ray Tube.

Fig. 172.—An X-Ray Tube.

The opposite end of the box is fitted with a covering of felt or velvet which shuts off the light around the eyes and nose when you look into the fluoroscope and hold it tightly against the face.

A fluoroscope may be purchased complete, or the platinum-barium-cyanide screen purchased separately and mounted on a box as shown in Figure 173.

The two anodes of the tube should be connected, and led to one terminal of a spark-coil capable of giving a spark at least one and one-half inches long. Another wire should be led from the cathode of the tube to the other terminal of the coil.

Fig. 173.—Fluoroscope.Fig. 173.—Fluoroscope.

Fig. 173.—Fluoroscope.

When it is desired to inspect any object, such as the hand, it must be held close to the screen of the fluoroscope and placed between the latter and the tube in the path of the X-rays. The X-rays are thrown forth from the tube at an angle of 45 degrees from the diagonal anode.

Look into the fluoroscope and it should appear to be filled with a green light. If not, the battery terminals connected to the primary of the coil should be reversed, so as to send the current through in the opposite direction.

The X-rays will cause the chemicals on the screen to light up and give forth a peculiar green light. If the hand is held against the screen, between the screen and the tube, the X-rays will pass through the hand and cast a shadow on the screen. They do not pass through the bones as easily as they do through the flesh and so will cast a shadow of the bones in the hand on the screen, and if you look closely you will be able to see the various joints, etc. The interrupter on the coil should be carefully adjusted so that the light does not flicker too much.

Fig. 174.—How to connect an X-Ray Tube to a Spark-Coil.Fig. 174.—How to connect an X-Ray Tube to a Spark-Coil.

Fig. 174.—How to connect an X-Ray Tube to a Spark-Coil.

If it is desired to take X-ray pictures, a fluoroscope is unnecessary.

Turn the tube around so that the X-rays point downward.

Shut the battery current off so that the tube is not in operation until everything else is ready.

Place an ordinary photographic plate, contained in an ordinary plate-holder, directly under the tube with the gelatin side of the plate upwards.

Place the hand flat on the plate and lower the tube until it is only about three inches above the hand. Then start the coil working so that the tube lights up and permit it to run for about fifteen minutes without removing the hand. Then turn the current off and develop the plate in a dark room.

It is possible to obtain a very good X-ray photograph of the hand in this manner. Photographs showing the skeleton of a mouse, nails in a board, coins in a purse, a bullet in a piece of wood, etc., are a few of the other objects which make interesting pictures.

An X-Ray Photograph of the hand taken with the Outfit shown in Figure 174.An X-Ray Photograph of the hand taken with the Outfit shown in Figure 174. The arrows point to injuries to the bone of the third finger near the middle Joint Resulting in a Stiff Joint.

An X-Ray Photograph of the hand taken with the Outfit shown in Figure 174. The arrows point to injuries to the bone of the third finger near the middle Joint Resulting in a Stiff Joint.

TRANSFORMERS

Back to Index Next