FIG. 196.—A phonograph. In this machine the cylinder is replaced by a revolving disk.FIG. 196.—A phonograph. In this machine the cylinder is replaced by a revolving disk.
280.Many animals possess the five senses, but only man possesses constructive, creative power, and is able to build on the information gained through the senses. It is the constructive, creative power which raises man above the level of the beast and enables him to devise and fashion wonderful inventions. Among the most important of his inventions are those which relate to electricity; inventions such as trolley car, elevator, automobile, electric light, the telephone, the telegraph. Bell, by his superior constructive ability, made possible the practical use of the telephone, and Marconi that of wireless telegraphy. To these inventions might be added many others which have increased the efficiency and production of the business world and have decreased the labor and strain of domestic life.
FIG. 197.—A simple electric cell.FIG. 197.—A simple electric cell.
281. Electricity as first Obtained by Man.Until modern times the only electricity known to us was that of the lightning flash, which man could neither hinder nor make. But in the year 1800, electricity in the form of a weak current was obtained by Volta of Italy in a very simple way; and even now our various electric batteries and cells are but a modification of that used by Volta and called a voltaic cell. A strip ofcopper and a strip of zinc are placed in a glass containing dilute sulphuric acid, a solution composed of oxygen, hydrogen, sulphur, and water. As soon as the plates are immersed in the acid solution, minute bubbles of gas rise from the zinc strip and it begins to waste away slowly. The solution gradually dissolves the zinc and at the same time gives up some of the hydrogen which it contains; but it has little or no effect on the copper, since there is no visible change in the copper strip.
If, now, the strips are connected by means of metal wires, the zinc wastes away rapidly, numerous bubbles of hydrogen pass over to the copper strip and collect on it, and a current of electricity flows through the connecting wires. Evidently, the source of the current is the chemical action between the zinc and the liquid.
Mere inspection of the connecting wire will not enable us to detect that a current is flowing, but there are various ways in which the current makes itself evident. If the ends of the wires attached to the strips are brought in contact with each other and then separated, a faint spark passes, and if the ends are placed on the tongue, a twinge is felt.
282. Experiments which grew out of the Voltaic Cell.Since chemical action on the zinc is the source of the current, it would seem reasonable to expect a current if the cell consisted of two zinc plates instead of one zinc plate and one copper plate. But when the copper strip is replaced by a zinc strip so that the cell consists of two similar plates, no current flows between them. In this case, chemical action is expended in heat rather than in the production of electricity and the liquid becomes hot. But if carbon and zinc are used, a current is again produced, the zinc dissolving away as before, and bubbles collecting on the carbon plate. By experiment it has been found that many different metals maybe employed in the construction of an electric cell; for example, current may be obtained from a cell made with a zinc plate and a platinum plate, or from a cell made with a lead plate and a copper plate. Then, too, some other chemical, such as bichromate of potassium, or ammonium chloride, may be used instead of dilute sulphuric acid.
Almost any two different substances will, under proper conditions, give a current, but the strength of the current is in some cases so weak as to be worthless for practical use, such as telephoning, or ringing a door bell. What is wanted is a strong, steady current, and our choice of material is limited to the substances which will give this result. Zinc and lead can be used, but the current resulting is weak and feeble, and for general use zinc and carbon are the most satisfactory.
283. Electrical Terms.The plates or strips used in making an electric cell are called electrodes; the zinc is called the negative electrode (-), and the carbon the positive electrode (+); the current is considered to flow through the wire from the + to the-electrode. As a rule, each electrode has attached to it a binding post to which wires can be quickly fastened.
The power that causes the current is called the electromotive force, and the value of the electromotive force, generally written E.M.F., of a cell depends upon the materials used.
When the cell consists of copper, zinc, and dilute sulphuric acid, the electromotive force has a definite value which is always the same no matter what the size or shape of the cell. But the E.M.F. has a decidedly different value in a cell composed of iron, copper, and chromic acid. Each combination of material has its own specific electromotive force.
284. The Disadvantage of a Simple Cell.When the poles of a simple voltaic cell are connected by a wire, the currentthus produced slowly diminishes in strength and, after a short time, becomes feeble. Examination of the cell shows that the copper plate is covered with hydrogen bubbles. If, however, these bubbles are completely brushed away by means of a rod or stick, the current strength increases, but as the bubbles again gather on the + electrode the current strength diminishes, and when the bubbles form a thick film on the copper plate, the current is too weak to be of any practical value. The film of bubbles weakens the current because it practically substitutes a hydrogen plate for a copper plate, and we saw in Section 282 that a change in any one of the materials of which a cell is composed changes the current.
This weakening of the current can be reduced mechanically by brushing away the bubbles as soon as they are formed; or chemically, by surrounding the copper plate with a substance which will combine with the free hydrogen and prevent it from passing onward to the copper plate.
FIG. 198.— The gravity cell.FIG. 198.— The gravity cell.
In practically all cells, the chemical method is used in preference to the mechanical one. The numerous types of cells in daily use differ chiefly in the devices employed for preventing the formation of hydrogen bubbles, or for disposing of them when formed. One of the best-known cells in which weakening of the current is prevented by chemical means is the so-called gravity cell.
285. The Gravity Cell.A large, irregular copper electrode is placed in the bottom of a jar (Fig. 198), andcompletely covered with a saturated solution of copper sulphate. Then a large, irregular zinc electrode is suspended from the top of the jar, and is completely covered with dilute sulphuric acid which does not mix with the copper sulphate, but floats on the top of it like oil on water. The hydrogen formed by the chemical action of the dilute sulphuric acid on the zinc moves toward the copper electrode, as in the simple voltaic cell. It does not reach the electrode, however, because, when it comes in contact with the copper sulphate, it changes places with the copper there, setting it free, but itself entering into the solution. The copper freed from the copper sulphate solution travels to the copper electrode, and is deposited on it in a clean, bright layer. Instead of a deposit of hydrogen there is a deposit of copper, and falling off in current is prevented.
The gravity cell is cheap, easy to construct, and of constant strength, and is in almost universal use in telegraphic work. Practically all small railroad stations and local telegraph offices use these cells.
FIG. 199.—A dry cell.FIG. 199.—A dry cell.
286. Dry Cells.The gravity cell, while cheap and effective, is inconvenient for general use, owing to the fact that it cannot be easily transported, and thedry cellhas largely supplanted all others, because of the ease with which it can be taken from place to place. This cell consists of a zinc cup, within which is a carbon rod; the space between the cup and rod is packed with a moist paste containing certain chemicals. The moist paste takes the place of the liquids used in other cells.
FIG. 200.—A battery of three cells.FIG. 200.—A battery of three cells.
287. A Battery of Cells. The electromotive force of one cell may not give a current strong enough to ring a door bell or to operate a telephone. But by using a number of cells, called a battery, the current may be increased to almost any desired strength. If three cells are arranged as in Figure 200, so that the copper of one cell is connected with the zinc of another cell, the electromotive force of the battery will be three times as great as the E.M.F. of a single cell. If four cells are arranged in the same way, the E.M.F. of the battery is four times as great as the E.M.F. of a single cell; when five cells are combined, the resulting E.M.F. is five times as great.
288. Heat. Any one who handles electric wires knows that they are more or less heated by the currents which flow through them. If three cells are arranged as in Figure 200 and the connecting wire is coarse, the heating of the wire is scarcely noticeable; but if a shorter wire of the same kind is used, the heat produced is slightly greater; and if the coarse wire is replaced by a short, fine wire, the heating of the wire becomes very marked. We are accustomed to say that a wire offers resistance to the flow of a current; that is, whenever a current meets resistance, heat is produced in much the same way as when mechanical motion meets an obstacle and spends its energy in friction. The flow of electricity along a wire can be compared to the flow of water through pipes: a small pipe offers a greater resistance to the flow of water than a large pipe; less water can be forced through a small pipe than through a large pipe, but the friction of the water against the sides of the small pipe is much greater than in the large one.
So it is with the electric current. In fine wires the resistance to the current is large and the energy of the battery is expended in heat rather than in current. If the heat thus produced is very great, serious consequences may arise; for example, the contact of a hot wire with wall paper or dry beams may cause fire. Insurance companies demand that the wires used in wiring a building for electric lights be of asize suitable to the current to be carried, otherwise they will not take the risk of insurance. The greater the current to be carried, the coarser is the wire required for safety.
289. Electric Stoves.It is often desirable to utilize the electric current for the production of heat. For example, trolley cars are heated by coils of wire under the seats. The coils offer so much resistance to the passage of a strong current through them that they become heated and warm the cars.
FIG. 201.—An electric iron on a metal stand.FIG. 201.—An electric iron on a metal stand.
Some modern houses are so built that electricity is received into them from the great plants where it is generated, and by merely turning a switch or inserting a plug, electricity is constantly available. In consequence, many practical applications of electricity are possible, among which are flatiron and toaster.
FIG. 202.—The fine wires are strongly heated by the current which flows through them.FIG. 202.—The fine wires are strongly heated by the current which flows through them.
Within the flatiron (Fig. 201), is a mass of fine wire coiled as shown in Figure 202; as soon as the iron is connected with the house supply of electricity, current flows through the fine wire which thus becomes strongly heated and gives off heat to the iron. The iron, when once heated, retains an even temperature as long as the current flows, and the laundress is, in consequence, free from the disadvantages of a slowly cooling iron, and of frequent substitution of a warm iron for a cold one. Electric irons are particularly valuable in summer, because they eliminate the necessity for a strong fire, and spare the housewife intense heat. In addition, the user is not confined to the laundry, but is free to seek the coolest part of the house, the onlyrequisite being an electrical connection.
FIG. 203.—Bread can be toasted by electricity.FIG. 203.—Bread can be toasted by electricity.
The toaster (Fig. 203) is another useful electrical device, since by means of it toast may be made on a dining table or at a bedside. The small electrical stove, shown in Figure 204, is similar in principle to the flatiron, but in it the heating coil is arranged as shown in Figure 205. To the physician electric stoves are valuable, since his instruments can be sterilized in water heated by the stove; and that without fuel or odor of gas.
A convenient device is seen in the heating pad (Fig. 206), a substitute for a hot water bag. Embedded in some soft thick substance are the insulated wires in which heat is to be developed, and over this is placed a covering of felt.
FIG. 204.—An electric stove.FIG. 204.—An electric stove.
290. Electric Lights.The incandescent bulbs which illuminate our buildings consist of a fine, hairlike thread inclosed in a glass bulb from which the air has been removed. When an electric current is sent through the delicate filament, it meets a strong resistance. The heat developed in overcoming the resistance is so great that it makes the filament a glowing mass. The absence of air prevents the filament from burning, and it merely glows and radiates the light.
FIG. 205.—The heating element in the electric stove.FIG. 205.—The heating element in the electric stove.
291. Blasting.Until recently, dynamiting was attended with serious danger, owing to the fact that the person who applied the torch to the fuse could not make a safe retreat before the explosion. Now a fine wire is inserted in the fuse, andwhen everything is in readiness, the ends of the wire are attached to the poles of a distant battery and the heat developed in the wire ignites the fuse.
FIG. 206.—An electric pad serves the same purpose as a hot water bag.FIG. 206.—An electric pad serves the same purpose as a hot water bag.
292. Welding of Metals.Metals are fused and welded by the use of the electric current. The metal pieces which are to be welded are pressed together and a powerful current is passed through their junction. So great is the heat developed that the metals melt and fuse, and on cooling show perfect union.
293. Chemical Effects.The Plating of Gold, Silver, and Other Metals.If strips of lead or rods of carbon are connected to the terminals of an electric cell, as in Figure 208, and are then dipped into a solution of copper sulphate, the strip in connection with the negative terminal of the cell soon becomes thinly plated with a coating of copper. If a solution of silver nitrate is used in place of the copper sulphate, the coating formed will be of silver instead of copper. So long as the current flows and there is any metal present in the solution, the coating continues to form on the negative electrode, and becomes thicker and thicker.
FIG. 207.—An incandescent electric bulb.FIG. 207.—An incandescent electric bulb.
The process by which metal is taken out of solution, as silver out of silver nitrate and copper out of copper sulphate, and is in turn deposited as a coating on another substance, is called electroplating. Anelectric current can separate a liquid into some of its various constituents and to deposit one of the metal constituents on the negative electrode.
FIG. 208.—Carbon rods in a solution of copper sulphate.FIG. 208.—Carbon rods in a solution of copper sulphate.
Since copper is constantly taken out of the solution of copper sulphate for deposit upon the negative electrode, the amount of copper remaining in the solution steadily decreases, and finally there is none of it left for deposit. In order to overcome this, the positive electrode should be made of the same metal as that which is to be deposited. The positive metal electrode gradually dissolves and replaces the metal lost from the solution by deposit and electroplating can continue as long as any positive electrode remains.
FIG. 209.—Plating spoons by electricity.FIG. 209.—Plating spoons by electricity.
Practically all silver, gold, and nickel plating is done in this way; machine, bicycle, and motor attachments are not solid, but are of cheaper material electrically plated with nickel. When spoons are to be plated, they are hung in a bath of silver nitrate side by side with a thick slab of pure silver, as in Figure 209. The spoons are connected with the negative terminal of the battery, while the slab of pure silver is connected with the positive terminal of the same battery. The length of time that the current flows determines thethickness of the plating.
294. How Pure Metal is obtained from Ore.When ore is mined, it contains in addition to the desired metal many other substances. In order to separate out the desired metal, the ore is placed in some suitable acid bath, and is connected with the positive terminal of a battery, thus taking the place of the silver slab in the last Section. When current flows, any pure metal which is present is dissolved out of the ore and is deposited on a convenient negative electrode, while the impurities remain in the ore or drop as sediment to the bottom of the vessel. Metals separated from the ore by electricity are called electrolytic metals and are the purest obtainable.
295. Printing.The ability of the electric current to decompose a liquid and to deposit a metal constituent has practically revolutionized the process of printing. Formerly, type was arranged and retained in position until the required number of impressions had been made, the type meanwhile being unavailable for other uses. Moreover, the printing of a second edition necessitated practically as great labor as did the first edition, the type being necessarily set afresh. Now, however, the type is set up and a mold of it is taken in wax. This mold is coated with graphite to make it a conductor and is then suspended in a bath of copper sulphate, side by side with a slab of pure copper. Current is sent through the solution as described in Section 293, until a thin coating of copper has been deposited on the mold. The mold is then taken from the bath, and the wax is replaced by some metal which gives strength and support to the thin copper plate. From this copper plate, which is an exact reproduction of the original type, many thousand copies can be printed. The plate can be preserved and used from time to time for later editions, and the original type can be put back into the cases and used again.
296. An Electric Current acts like a Magnet.In order to understand the action of the electric bell, we must consider a third effect which an electric current can cause. Connect some cells as shown in Figure 200 and close the circuit through a stout heavy copper wire, dipping a portion of the wire into fine iron filings. A thick cluster of filings will adhere to the wire (Fig. 210), and will continue to cling to it so long as the current flows. If the current is broken, the filings fall from the wire, and only so long as the current flows through the wire does the wire have power to attract iron filings. An electric current makes a wire equivalent to a magnet, giving it the power to attract iron filings.
FIG. 210.—A wire carrying current attracts iron filings.FIG. 210.—A wire carrying current attracts iron filings.
FIG. 211.—A loosely wound coil of wire.FIG. 211.—A loosely wound coil of wire.
Although such a straight current bearing wire attracts iron filings, its power of attraction is very small; but its magnetic strength can be increased by coiling as in Figure 211. Such an arrangement of wire is known as a helix or solenoid, and is capable of lifting or pulling larger and more numerous filings and even good-sized pieces of iron, such as tacks. Filings do not adhere to the sides of the helix, but they cling in clusters to the endsof the coil. This shows that the ends of the helix have magnetic power but not the sides.
If a soft iron nail (Fig. 212) or its equivalent is slipped within the coil, the lifting and attractive power of the coil is increased, and comparatively heavy weights can be lifted.
FIG. 212.—Coil and soft iron rod.FIG. 212.—Coil and soft iron rod.
A coil of wire traversed by an electric current and containing a core of soft iron has the power of attracting and moving heavy iron objects; that is, it acts like a magnet. Such an arrangement is called an electromagnet. As soon as the current ceases to flow, the electromagnet loses its magnetic power and becomes merely iron and wire without magnetic attraction.
If many cells are used, the strength of the electromagnet is increased, and if the coil is wound closely, as in Figure 213, instead of loosely, as in Figure 211, the magnetic strength is still further increased. The strength of any electromagnet depends upon the number of coils wound on the iron core and upon the strength of the current which is sent through the coils.
FIG. 213.—An electromagnet.FIG. 213.—An electromagnet.
FIG. 214.—A horseshoe electromagnet is powerful enough to support heavy weights.FIG. 214.—A horseshoe electromagnet is powerful enough to support heavy weights.
To increase the strength of the electromagnet still further, the so-called horseshoe shape is used (Fig. 214). In such an arrangement there is practically the strength of two separate electromagnets.
297. The Electric Bell.The ringing of the electric bell is due to the attractive power of an electromagnet. By the pushing of a button (Fig. 215) connection is made with a battery, and current flows through the wire wound on the iron spools, and further to the screwPwhich presses against the soft iron strip or armatureS; and fromSthecurrent flows back to the battery. As soon as the current flows, the coils become magnetic and attract the soft iron armature, drawing it forward and causing the clapper to strike the bell. In this position,Sno longer touches the screwP, and hence there is no complete path for the electricity, and the current ceases. But the attractive, magnetic power of the coils stops as soon as the current ceases; hence there is nothing to hold the armature down, and it flies back to its former position. In doing this, however, the armature makes contact atPthrough the spring, and the current flows once more; as a result the coils again become magnets, the armature is again drawn forward, and the clapper again strikes the bell. But immediately afterwards the armature springs backward and makes contact atPand the entire operation is repeated. So long as we press the button this process continues producing what sounds like a continuous jingle; in reality the clapper strikes the bell every time a current passes through the electromagnet.
FIG. 215.—The electric bell.FIG. 215.—The electric bell.
298. The Push Button.The push button is an essential part of every electric bell, because without it the bell either would not ring at all, or would ring incessantly until the cell was exhausted. When the push button is free, as in Figure 216, the cell terminals are not connected in an unbroken path, and hence the current does not flow. When, however, the button is pressed, the current has a complete path, provided there is the proper connection atS. That is, the pressure on the push button permits current to flow to the bell. The flow of this current then depends solely upon the connection atS, which is alternately made and broken, and in this way produces sound.
FIG. 216.—Push button.FIG. 216.—Push button.
The sign "Bell out of order" is usually due to the fact that the battery is either temporarily or permanently exhausted. In warm weather the liquid in the cell may dry up and cause stoppage of the current. If fresh liquid is poured into the vessel so that the chemical action of the acid on the zinc is renewed, the current again flows. Another explanation of an out-of-order bell is that the liquid may have eaten up all the zinc; if this is the case, the insertion of a fresh strip of zinc will remove the difficulty and the current will flow. If dry cells are used, there is no remedy except in the purchase of new cells.
299. How Electricity may be lost to Use.In the electric bell, we saw that an air gap at the push button stopped the flow of electricity. If we cut the wire connecting the poles of a battery, the current ceases because an air gap intervenes and electricity does not readily pass through air. Many substances besides air stop the flow of electricity. If a strip of glass, rubber, mica, or paraffin is introduced anywhere in a circuit, the current ceases. If a metal is inserted in the gap,the current again flows. Substances which, like an air gap, interfere with the flow of electricity are called non-conductors, or, more commonly, insulators. Substances which, like the earth, the human body, and all other moist objects, conduct electricity are conductors. If the telephone and electric light wires in our houses were not insulated by a covering of thread, or cloth, or other non conducting material, the electricity would escape into surrounding objects instead of flowing through the wire and producing sound and light.
In our city streets, the overhead wires are supported on glass knobs or are closely wrapped, in order to prevent the escape of electricity through the poles to the ground. In order to have a steady, dependable current, the wire carrying the current must be insulated.
Lack of insulation means not only the loss of current for practical uses, but also serious consequences in the event of the crossing of current-bearing wires. If two wires properly insulated touch each other, the currents flow along their respective wires unaltered; if, however, two uninsulated wires touch, some of the electricity flows from one to the other. Heat is developed as a result of this transference, and the heat thus developed is sometimes so great that fire occurs. For this reason, wires are heavily insulated and extra protection is provided at points where numerous wires touch or cross.
Conductors and insulators are necessary to the efficient and economic flow of a current, the insulator preventing the escape of electricity and lessening the danger of fire, and the conductor carrying the current.
300. The Telegraph.Telegraphy is the process of transmitting messages from place to place by means of an electric current. The principle underlying the action of the telegraph is the principle upon which the electric bell operates;namely, that a piece of soft iron becomes a magnet while a current flows around it, but loses its magnetism as soon as the current ceases.
In the electric bell, the electromagnet, clapper, push button, and battery are relatively near,—usually all are located in the same building; while in the telegraph the current may travel miles before it reaches the electromagnet and produces motion of the armature.
FIG. 217.—Diagram of the electric telegraph.FIG. 217.—Diagram of the electric telegraph.
The fundamental connections of the telegraph are shown in Figure 217. If the keyKis pressed down by an operator in Philadelphia, the current from the battery (only one cell is shown for simplicity) flows through the line to New York, passes through the electromagnetM, and thence back to Philadelphia. As long as the keyKis pressed down, the coilMacts as a magnet and attracts and holds fast the armatureA; but as soon asKis released, the current is broken,Mloses its magnetism, and the armature is pulled back by the springD. By a mechanical device, tape is drawn uniformly under the light markerPattached to the armature. IfKis closed for but a short time, the armature is drawn down for but a short interval, and the marker registers a dot on the tape. IfKis closed for a longer time, a short dash is made by the marker, and, in general, the length of time thatKis closed determines the length of the marks recorded on the tape.The telegraphic alphabet consists of dots and dashes and their various combinations, and hence an interpretation of the dot and dash symbols recorded on the tape is all that is necessary for the receiving of a telegraphic message.
The Morse telegraphic code, consisting of dots, dashes, and spaces, is given in Figure 218.
FIG. 218.—The Morse telegraphic code.FIG. 218.—The Morse telegraphic code.
The telegraph is now such a universal means of communication between distant points that one wonders how business was conducted before its invention in 1832 by S.F.B. Morse.
FIG. 219.—The sounder.FIG. 219.—The sounder.
301. Improvements.The Sounder.Shortly after the invention of telegraphy, operators learned that they could read the message by the click of the marker against a metal rod which took the place of the tape. In practically all telegraph offices of the present day the old-fashioned tape is replaced by the sounder, shown in Figure 219. When current flows, a lever,L, is drawn down by the electromagnet and strikes against a solid metal piece with a click; when the current is broken, the lever springs upward, strikes another metal piece and makes a different click. It is clear that the working of the key which starts and stops the current in this line will beimitated by the motion and the resulting clicks of the sounder. By means of these varying clicks of the sounder, the operator interprets the message.
FIG. 220.—Diagram of a modern telegraph system.FIG. 220.—Diagram of a modern telegraph system.
The Relay.When a telegraph line is very long, the resistance of the wire is great, and the current which passes through the electromagnet is correspondingly weak, so feeble indeed that the armature must be made very thin and light in order to be affected by the makes and breaks in the current. The clicks of an armature light enough to respond to the weak current of a long wire are too faint to be recognized by the ear, and hence in such long circuits some device must be introduced whereby the effect is increased. This is usually done by installing at each station a local battery and a very delicate and sensitive electromagnet called therelay. Under these conditions the current of the main line is not sent through the sounder, but through the relay which opens and closes a local battery in connection with the strongsounder. For example, the relay is so arranged that current from the main line runs through it exactly as it runs throughMin Figure 217. When current is made, the relay attracts an armature, which thereby closes a circuit in a local battery and thus causes a click of the sounder. When the current in the main line is broken, the relay loses its magnetic attraction, its armature springs back, connection is broken in the local circuit, and the sounder responds by allowing its armature to spring back with a sharp sound.
302. The Earth an Important Part of a Telegraphic System.We learned in Section 299 that electricity could flow through many different substances, one of which was the earth. In all ordinary telegraph lines, advantage is taken of this fact to utilize the earth as a conductor and to dispense with one wire. Originally two wires were used, as in Figure 217; then it was found that a railroad track could be substituted for one wire, and later that the earth itself served equally well for a return wire. The present arrangement is shown in Figure 220, where there is but one wire, the circuit being completed by the earth. No fact in electricity seems more marvelous than that the thousands of messages flashing along the wires overhead are likewise traveling through the ground beneath. If it were not for this use of the earth as an unfailing conductor, the network of overhead wires in our city streets would be even more complex than it now is.
303. Advances in Telegraphy.The mechanical improvements in telegraphy have been so rapid that at present a single operator can easily send or receive forty words a minute. He can telegraph more quickly than the average person can write; and with a combination of the latest improvements the speed can be enormously increased. Recently, 1500 words were flashed from New York to Boston over asingle wire in one second.
In actual practice messages are not ordinarily sent long distances over a direct line, but are automatically transferred to new lines at definite points. For example, a message from New York to Chicago does not travel along an uninterrupted path, but is automatically transferred at some point, such as Lancaster, to a second line which carries it on to Pittsburgh, where it is again transferred to a third line which takes it farther on to its destination.
304. In the twelfth century, there was introduced into Europe from China a simple instrument which changed journeying on the sea from uncertain wandering to a definite, safe voyage. This instrument was the compass (Fig. 221), and because of the property of the compass needle (a magnet) to point unerringly north and south, sailors were able to determine directions on the sea and to steer for the desired point.
FIG. 221.—The compass.FIG. 221.—The compass.
Since an electric current is practically equivalent to a magnet (Section 296), it becomes necessary to know the most important facts relative to magnets, facts simple in themselves but of far-reaching value and consequences in electricity. Without a knowledge of the magnetic characteristics of currents, the construction of the motor would have been impossible, and trolley cars, electric fans, motor boats, and other equally well-known electrical contrivances would be unknown.
305. The Attractive Power of a Magnet. The magnet best known to us all is the compass needle, but for conveniencewe will use a magnetic needle in the shape of a bar larger and stronger than that employed in the compass. If we lay such a magnet on a pile of iron filings, it will be found on lifting the magnet that the filings cling to the ends in tufts, but leave it almost bare in the center (Fig. 222). The points of attraction at the two ends are called the poles of the magnet.
FIG. 222.—A magnet.FIG. 222.—A magnet.
If a delicately made magnet is suspended as in Figure 223, and is allowed to swing freely, it will always assume a definite north and south position. The pole which points north when the needle is suspended is called the north pole and is markedN, while the pole which points south when the needle is suspended is called the south pole and is markedS.
A freely suspended magnet points nearly north and south.
A magnet has two main points of attraction called respectively the north and south poles.
FIG. 223.—The magnetic needle.FIG. 223.—The magnetic needle.
306. The Extent of Magnetic Attraction. If a thin sheet of paper or cardboard is laid over a strong, bar-shaped magnet and iron filings are then gently strewn on the paper, the filings clearly indicate the position of the magnet beneath, and if the cardboard is gently tapped, the filings arrange themselves as shown in Figure 224. If the paper is held some distance above the magnet, the influence on the filings is less definite, and finally, if the paper is held very far away, thefilings do not respond at all, but lie on the cardboard as dropped.
The magnetic power of a magnet, while not confined to the magnet itself, does not extend indefinitely into the surrounding region; the influence is strong near the magnet, but at a distance becomes so weak as to be inappreciable. The region around a magnet through which its magnetic force is felt is called the field of force, or simply the magnetic field, and the definite lines in which the filings arrange themselves are called lines of force.
FIG. 224.—Iron filings scattered over a magnet arrange themselves in definite lines.FIG. 224.—Iron filings scattered over a magnet arrange themselves in definite lines.
The magnetic power of a magnet is not limited to the magnet, but extends to a considerable distance in all directions.
307. The Influence of Magnets upon Each Other. If while our suspended magnetic needle is at rest in its characteristic north-and-south direction another magnet is brought near, the suspended magnet is turned; that is, motion is produced (Fig. 225). If the north pole of the free magnet is brought toward the south pole of the suspended magnet, the latter moves in such a way that the two polesNandSare as close together as possible. If the north pole of the free magnetis brought toward the north pole of the suspended magnet, the latter moves in such a way that the two polesNandNare as far apart as possible. In every case that can be tested, it is found that a north pole repels a north pole, and a south pole repels a south pole; but that a north and a south pole always attract each other.
FIG. 225.—A south pole attracts a north pole.FIG. 225.—A south pole attracts a north pole.
The main facts relative to magnets may be summed up as follows:—
a. A magnet points nearly north and south if it is allowed to swing freely.
b. A magnet contains two unlike poles, one of which persistently points north, and the other of which as persistently points south, if allowed to swing freely.
c. Poles of the same name repel each other; poles of unlike name attract each other.
d. A magnet possesses the power of attracting certain substances, like iron, and this power of attraction is not limited to the magnet itself but extends into the region around the magnet.
308. Magnetic Properties of an Electric Current. If a current-bearing wire is really equivalent in its magnetic powers to a magnet, it must possess all of the characteristics mentioned in the preceding Section. We saw in Section 296 that a coiled wire through which current was flowing would attract iron filings at the two ends of the helix. That a coil through which current flows possesses the characteristicsa,b,c, anddof a magnet is shown as follows:—
a,b. If a helix marked at one end with a red string is arranged so that it is free to rotate and a strong current issent through it, the helix will immediately turn and face about until it points north and south. If it is disturbed from this position, it will slowly swing back until it occupies its characteristic north and south position. The end to which the string is attached will persistently point either north or south. If the current is sent through the coil in the opposite direction, the two poles exchange positions and the helix turns until the new north pole points north.