The method now generally practiced is to cast a frame of lead, with raised right-angled ribs on each side, thus forming little depressed squares, or to punch a lead plate full of holes, which squares or holes are then filled with a pasty mixture of red oxide of lead in positive plates, and with litharge in negatives. In a form called the chloride battery, instead of cementing lead oxide paste into or against a lead framing in order to obtain the necessary active material, the latter is obtained by a strictly chemical process.
Fig. 82 shows a storage cell with plates, etc., contained in a glass jar. Fig. 83 shows a cell of 41 plates, set up in a lead-lined wood tank. Fig. 84 shows three cells joined in series. Many storage cells are used in central electric light stations to help the dynamos during the "rush" hours at night. They are charged during the day when the load on the dynamos is not heavy.
Fig. 85 shows another form of storage cell containing a number of plates.
87. The Uses of Storage Batteries are almost numberless. The current can be used for nearly everything for which a constant current is adapted, the following being some of its applications: Carriage propulsion; electric launch propulsion; train lighting; yacht lighting; carriage lighting; bicycle lighting; miners' lamps; dental, medical, surgical, and laboratory work; phonographs; kinetoscopes; automaton pianos; sewing-machine motors; fan motors; telegraph; telephone; electric bell; electric fire-alarm; heat regulating; railroad switch and signal apparatus.
By the installing of a storage plant many natural but small sources of power may be utilized in furnishing light and power; sources which otherwise are not available, because not large enough to supply maximum demands. The force of the tides, of small water powers from irrigating ditches, and even of the wind, come under this heading.
As a regulator of pressure, in case of fluctuations in the load, the value of a storage plant is inestimable. These fluctuations of load are particularly noticeable in electric railway plants, where the demand is constantly rising and falling, sometimes jumping from almost nothing to the maximum, and vice versa, in a few seconds. If for no other reason than the prevention of severe strain on the engines and generators, caused by these fluctuations of demand, a storage plant will be valuable.
CHAPTER X.
HOW ELECTRICITY IS GENERATED BY HEAT.
88. Thermoelectricity is the name given to electricity that is generated by heat. If a strip of iron, I, be connected between two strips of copper, C C, these being joined by a copper wire, C W, we shall have an arrangement that will generate a current when heated at either of the junctions between C and I. When it is heated at A the current will flow as shown by arrows, from C to I. If we heat at B, the current will flow in the opposite direction through the metals, although it will still go from C to I as before. Such currents are called thermoelectric currents.
Different pairs of metals produce different results. Antimony and bismuth are generally used, because the greatest effect is produced by them. If the end of a strip of bismuth be soldered to the end of a similar strip of antimony, and the free ends be connected to a galvanometer of low resistance, the presence of a current will be shown when the point of contact becomes hotter than the rest of the circuit. The current will flow from bismuth to antimony across the joint. By cooling the juncture below the temperature of the rest of the circuit, a current will be produced in the opposite direction to the above. The energy of the current is kept up by the heat absorbed, just as it is kept up by chemical action in the voltaic cell.
89. Peltier Effect. If an electric current be passed through pairs of metals, the parts at the junction become slightly warmer or cooler than before, depending upon the direction of the current. This action is really the reverse of that in which currents are produced by heat.
90. Thermopiles. As the E.M.F. of the current produced by a single pair of metals is very small, several pairs are usually joined in series, so that the different currents will help each other by flowing in the same direction. Such combinations are called thermoelectric piles, or simply thermopiles.
Fig. 87 shows such an arrangement, in which a large number of elements are placed in a small space. The junctures are so arranged that the alternate ones come together at one side.
Fig. 88 shows a thermopile connected with a galvanometer. The heat of a match, or the cold of a piece of ice, will produce a current, even if held at some distance from the thermopile. The galvanometer should be a short-coil astatic one. (See "Study," Chapter XXIV., for experiments and home-made thermopile.)
CHAPTER XI.
MAGNETIC EFFECTS OF THE ELECTRIC CURRENT.
91. Electromagnetism is the name given to magnetism that is developed by electricity. We have seen that if a magnetic needle be placed in the field of a magnet, its N pole will point in the direction taken by the lines of force as they pass from the N to the S pole of the magnet.
92. Lines of Force about a Wire. When a current passes through a wire, the magnetic needle placed over or under it tends to take a position at right angles to the wire. Fig. 89 shows such a wire and needle, and how the needle is deflected; it twists right around from its N and S position as soon as the current begins to flow. This shows that the lines of force pass around the wire and not in the direction of its length. The needle does not swing entirely perpendicular to the wire, that is, to the E and W line, because the earth is at the same time pulling its N pole toward the N.
Fig. 90 shows a bent wire through which a current passes from C to Z. If you look along the wire from C toward the points A and B, you will see that under the wire the lines of force pass to the left. Looking along the wire from Z toward D you will see that the lines of force pass opposite to the above, as the current comes toward you. This is learned by experiment. (See "Study," Exp. 152, § 385, etc.)
Rule. Hold the right hand with the thumb extended (Fig. 89) and with the fingers pointing in the direction of the current, the palm being toward the needle and on the opposite side of the wire from the needle. The north-seeking pole will then be deflected in the direction in which the thumb points.
93. Current Detectors. As there is a magnetic field about a wire when a current passes through it, and as the magnetic needle is affected, we have a means of detecting the presence of a current. When the current is strong it is simply necessary to let it pass once over or under a needle; when it is weak, the wire must pass several times above and below the needle, Fig. 91, to give the needle motion. (See "Apparatus Book," Chapter XIII., for home-made detectors.)
94. Astatic Needles and Detectors. By arranging two magnetized needles with their poles opposite each other, Fig. 92, an astatic needle is formed. The pointing-power is almost nothing, although their magnetic fields are retained. This combination is used to detect feeble currents. In the ordinary detector, the tendency of the needle to point to the N and S has to be overcome by the magnetic field about the coil before the needle can be moved; but in the astatic detector and galvanoscope this pointing-power is done away with. Fig. 93 shows a simple astatic galvanoscope. Fig. 67 shows an astatic galvanometer for measuring weak currents.
95. Polarity of Coils. When a current of electricity passes through a coil of wire, the coil acts very much like a magnet, although no iron enters into its construction. The coil becomes magnetized by the electric current, lines of force pass from it into the air, etc. Fig. 94 shows a coil connected to copper and zinc plates, so arranged with cork that the whole can float in a dish of dilute sulphuric acid. The current passes as shown by the arrows, and when the N pole of a magnet is brought near the right-hand end, there is a repulsion, showing that that end of the coil has a N pole.
Rule. When you face the right-hand end of the coil, the current is seen to pass around it in an anti-clockwise direction; this produces a N pole. When the current passes in a clockwise direction a S pole is produced.
96. Electromagnets. A coil of wire has a stronger field than a straight wire carrying the same current, because each turn adds its field to the fields of the other turns. By having the central part of the coil made of iron, or by having the coil of insulated wire wound upon an iron core, the strength of the magnetic field of the coil is greatly increased.
Lines of force do not pass as readily through air as through iron; in fact, lines of force will go out of their way to go through iron. With a coil of wire the lines of force pass from its N pole through the air on all sides of the coil to its S pole; they then pass through the inside of the coil and through the air back to the N pole. When the resistance to their passage through the coil is decreased by the core, the magnetic field is greatly strengthened, and we have an electromagnet.
The coil of wire temporarily magnetizes the iron core; it can permanently magnetize a piece of steel used as a core. (See "Study," Chapter XXII., for experiments.)
97. Forms of Electromagnets. Fig. 95 shows a straight, or bar electromagnet. Fig. 96 shows a simple form of horseshoe electromagnet. As this form is not easily wound, the coils are generally wound on two separate cores which are then joined by a yoke. The yoke merely takes the place of the curved part shown in Fig. 96. In Fig. 97 is shown the ordinary form of horseshoe electromagnet used for all sorts of electrical instruments. (See "Apparatus Book," Chapter IX., for home-made electromagnets.)
98. Yokes and Armatures. In the horseshoe magnet there are two poles to attract and two to induce. The lines of force pass through the yoke on their way from one core to the other, instead of going through the air. This reduces the resistance to them. If we had no yoke we should simply have two straight electromagnets, and the resistance to the lines of force would be so great that the total strength would be much reduced. Yokes are made of soft iron, as well as the cores and armature. The armature, as with permanent horseshoe magnets, is strongly drawn toward the poles. As soon as the current ceases to flow, the attraction also ceases.
Fig. 96
| Fig. 97.
|
Beautiful magnetic figures can be made with horseshoe magnets. Fig. 98 shows that the coils must be joined so that the current can pass around the cores in opposite directions to make unlike poles. (See "Study," Exp. 164 to 173.)
CHAPTER XII.
HOW ELECTRICITY IS GENERATED BY INDUCTION.
99. Electromagnetic Induction. We have seen that a magnet has the power to act through space and induce another piece of iron or steel to become a magnet. A charge of static electricity can induce a charge upon another conductor. We have now to see how a current of electricity in one conductor can induce a current in another conductor, not in any way connected with the first, and how a magnet and a coil can generate a current.
Fig. 99.
| Fig. 100.
|
100. Current from Magnet and Coil. If a bar magnet, Fig. 99, be suddenly thrust into a hollow coil of wire, a momentary current of electricity will be generated in the coil. No current passes when the magnet and coil are still; at least one of them must be in motion. Such a current is said to be induced, and is an inverse one when the magnet is inserted, and a direct one when the magnet is withdrawn from the coil.
101. Induced Currents and Lines of Force. Permanent magnets are constantly sending out thousands of lines of force. Fig. 100 shows a bar magnet entering a coil of wire; the number of lines of force is increasing, and the induced current passes in an anti-clockwise direction when looking down into the coil along the lines of force. This produces an indirect current. If an iron core be used in the coil, the induced current will be greatly strengthened.
It takes force to move a magnet through the center of a coil, and it is this work that is the source of the induced current. We have, in this simple experiment, the key to the action of the dynamo and other electrical machines.
102. Current from two Coils. Fig. 101 shows two coils of wire, the smaller being connected to a cell, the larger to a galvanometer. By moving the small coil up and down inside of the large one, induced currents are generated, first in one direction and then in the opposite. We have here two entirely separate circuits, in no way connected. The primary current comes from the cell, while the secondary current is an induced one. By placing a core in the small coil of Fig. 101, the induced current will be greatly strengthened.
It is not necessary to have the two coils so that one or both of them can move. They may be wound on the same core, or otherwise arranged as in the induction coil. (See "Study," Chapter XXV., for experiments on induced currents.)
CHAPTER XIII.
HOW THE INDUCTION COIL WORKS.
103. The Coils. We saw, § 102, that an induced current was generated when a current-carrying coil, Fig. 101, was thrust into another coil connected with a galvanometer. The galvanometer was used merely to show the presence of the current. The primary coil is the one connected with the cell; the other one is called the secondary coil.
When a current suddenly begins to flow through a coil, the effect upon a neighboring coil is the same as that produced by suddenly bringing a magnet near it; and when the current stops, the opposite effect is produced. It is evident, then, that we can keep the small coil of Fig. 101 with its core inside of the large coil, and generate induced currents by merely making and breaking the primary circuit.
We may consider that when the primary circuit is closed, the lines of force shoot out through the turns of the secondary coil just as they do when a magnet or a current-carrying coil is thrust into it. Upon opening the circuit, the lines of force cease to exist; that is, we may imagine them drawn in again.
104. Construction. Fig. 102 shows one form of home-made induction coil, given here merely to explain the action and connections. Nearly all induction coils have some form of automatic current interrupter, placed in the primary circuit, to rapidly turn the current off and on.
Details of Figs. 102 and 103. Wires 5 and 6 are the ends of the primary coil, while wires 7 and 8 are the terminals of the secondary coil. The primary coil is wound on a bolt which serves as the core, and on this coil is wound the secondary which consists of many turns of fine wire. The wires from a battery should be joined to binding-posts W and X, and the handles, from which the shock is felt, to Y and Z. Fig. 103 shows the details of the interrupter.
If the current from a cell enters at W, it will pass through the primary coil and out at X, after going through 5, R, F, S I, B, E and C. The instant the current passes, the bolt becomes magnetized; this attracts A, which pulls B away from the end of S I, thus automatically opening the circuit. B at once springs back to its former position against SI, as A is no longer attracted; the circuit being closed, the operation is rapidly repeated.
A condenser is usually connected to commercial forms. It is placed under the wood-work and decreases sparking at the interrupter. (See "Apparatus Book," Chapter XI., for home-made induction coils.)
Fig. 104 shows one form of coil. The battery wires are joined to the binding-posts at the left. The secondary coil ends in two rods, and the spark jumps from one to the other. The interrupter and a switch are shown at the left.
Fig. 105 shows a small coil for medical purposes. A dry cell is placed under the coil and all is included in a neat box. The handles form the terminals of the secondary coil.
105. The Currents. It should be noted that the current from the cell does not get into the secondary coil. The coils are thoroughly insulated from each other. The secondary current is an induced one, its voltage depending upon the relative number of turns of wire there are in the two coils. (See Transformers.) The secondary current is an alternating one; that is, it flows in one direction for an instant and then immediately reverses its direction. The rapidity of the alternations depends upon the speed of the interrupter. Coils are made that give a secondary current with an enormous voltage; so high, in fact, that the spark will pass many inches, and otherwise act like those produced by static electric machines.
106. Uses of Induction Coils. Gas-jets can be lighted at a distance with the spark from a coil, by extending wires from the secondary coil to the jet. Powder can be fired at a distance, and other things performed, when a high voltage current is needed. Its use in medicine has been noted. It is largely used in telephone work. Of late, great use has been made of the secondary current in experiments with vacuum-tubes, X-ray work, etc.
CHAPTER XIV.
THE ELECTRIC TELEGRAPH, AND HOW IT SENDS
MESSAGES.
107. The Complete Telegraph Line consists of several instruments, switches, etc., etc., but its essential parts are: The Line, or wire, which connects the different stations; the Transmitter or Key; the Receiver or Sounder, and the Battery or Dynamo.
108. The Line is made of strong copper, iron, or soft steel wire. To keep the current in the line it is insulated, generally upon poles, by glass insulators. For very short lines two wires can be used, the line wire and the return; but for long lines the earth is used as a return, a wire from each end being joined to large metal plates sunk in the earth.
109. Telegraph Keys are merely instruments by which the circuit can be conveniently and rapidly opened or closed at the will of the operator. An ordinary push-button may be used to turn the current off and on, but it is not so convenient as a key.
Fig. 106 shows a side view of a simple key which can be put anywhere in the circuit, one end of the cut wire being attached to X and the other to Y. By moving the lever C up and down according to a previously arranged set of signals, a current will be allowed to pass to a distant station. As X and Y are insulated from each other, the current can pass only when C presses against Y.
Fig. 107 shows a regular key, with switch, which is used to allow the current to pass through the instrument when receiving a message.
110. Telegraph Sounders receive the current from some distant station, and with its electromagnet produce sounds that can be translated into messages.
Fig. 108 shows simply an electromagnet H, the coil being connected in series with a key K and a cell D C. The key and D C are shown by a top view. The lever of K does not touch the other metal strap until it is pressed down. A little above the core of H is held a strip of iron, on armature I. As soon as the circuit is closed at K, the current rushes through the circuit, and the core attracts I making a distinct click. As soon as K is raised, I springs away from the core, if it has been properly held. In regular instruments a click is also made when the armature springs back again.
The time between the two clicks can be short or long, to represent dots or dashes, which, together with spaces, represent letters. (For Telegraph Alphabet and complete directions for home-made keys, sounders, etc., see "Apparatus Book," Chapter XIV.)
Fig. 109 shows a form of home-made sounder. Fig. 110 shows one form of telegraph sounder. Over the poles of the horseshoe electromagnet is an armature fixed to a metal bar that can rock up and down. The instant the current passes through the coils the armature comes down until a stop-screw strikes firmly upon the metal frame, making the down click. As soon as the distant key is raised, the armature is firmly pulled back and another click is made. The two clicks differ in sound, and can be readily recognized by the operator.
111. Connections for Simple Line. Fig. 111 shows complete connections for a home-made telegraph line. The capital letters are used for the right side, R, and small letters for the left side, L. Gravity cells, B and b, are used. The sounders, S and s, and the keys, K and k, are shown by a top view. The broad black lines of S and s represent the armatures which are directly over the electromagnets. The keys have switches, E and e.
The two stations, R and L, may be in the same room, or in different houses. The return wire, R W, passes from the copper of b to the zinc of B. This is important, as the cells must help each other; that is, they are in series. The line wire, L W, passes from one station to the other, and the return may be through the wire, R W, or through the earth; but for short lines a wire is best.
112. Operation of Simple Line. Suppose two boys, R (right) and L (left) have a line. Fig. 111 shows that R's switch, E, is open, while e is closed. The entire circuit, then, is broken at but one point. As soon as R presses his key, the circuit is closed, and the current from both cells rushes around from B, through K, S, L W, s, k, b, R W, and back to B. This makes the armatures of S and s come down with a click at the same time. As soon as the key is raised, the armatures lift and make the up-click. As soon as R has finished, he closes his switch E. As the armatures are then held down, L knows that R has finished, so he opens his switch e, and answers R. Both E and e are closed when the line is not in use, so that either can open his switch at any time and call up the other. Closed circuit cells must be used for such lines. On very large lines dynamos are used to furnish the current.
113. The Relay. Owing to the large resistance of long telegraph lines, the current is weak when it reaches a distant station, and not strong enough to work an ordinary sounder. To get around this, relays are used; these are very delicate instruments that replace the sounder in the line wire circuit. Their coils are usually wound with many turns of fine wire, so that a feeble current will move its nicely adjusted armature. The relay armature merely acts as an automatic key to open and close a local circuit which includes a battery and sounder. The line current does not enter the sounder; it passes back from the relay to the sending station through the earth.
Fig. 112 gives an idea of simple relay connections. The key K, and cell D C, represent a distant sending station. E is the electromagnet of the relay, and R A is its armature. L W and R W represent the line and return wires. R A will vibrate toward E every time K is pressed, and close the local circuit, which includes a local battery, L B, and a sounder. It is evident that as soon as K is pressed the sounder will work with a good strong click, as the local battery can be made as strong as desired.
Fig. 113 shows a regular instrument which opens and closes the local circuit at the top of the armature.
114. Ink Writing Registers are frequently used instead of sounders. Fig. 114 shows a writing register that starts itself promptly at the opening of the circuit, and stops automatically as soon as the circuit returns to its normal condition. A strip of narrow paper is slowly pulled from the reel by the machine, a mark being made upon it every time the armature of an inclosed electromagnet is attracted. When the circuit is simply closed for an instant, a short line, representing a dot, is made.
Registers are built both single pen and double pen. In the latter case, as the record of one wire is made with a fine pen, and the other with a coarse pen, they can always be identified. The record being blocked out upon white tape in solid black color, in a series of clean-cut dots and dashes, it can be read at a glance, and as it is indelible, it may be read years afterward. Registers are made for local circuits, for use in connection with relays, or for direct use on main lines, as is usually desirable in fire-alarm circuits.
CHAPTER XV.
THE ELECTRIC BELL AND SOME OF ITS USES.
115. Automatic Current Interrupters are used on most common bells, as well as on induction coils, etc. (See § 104.) Fig. 115 shows a simple form of interrupter. The wire 1, from a cell D C, is joined to an iron strip I a short distance from its end. The other wire from D C passes to one end of the electromagnet coil H. The remaining end of H is placed in contact with I as shown, completing the circuit. As soon as the current passes, I is pulled down and away from the upper wire 2, breaking the circuit. I, being held by its left-hand end firmly in the hand, immediately springs back to its former position, closing the circuit again. This action is repeated, the rapidity of the vibrations depending somewhat upon the position of the wires on I. In regular instruments a platinum point is used where the circuit is broken; this stands the sparking when the armature vibrates.
116. Electric Bells may be illustrated by referring to Fig. 116, which shows a circuit similar to that described in § 115, but which also contains a key K, in the circuit. This allows the circuit to be opened and closed at a distance from the vibrating armature. The circuit must not be broken at two places at the same time, so wires should touch at the end of I before pressing K. Upon pressing K the armature I will vibrate rapidly. By placing a small bell near the end of the vibrating armature, so that it will be struck by I at each vibration, we should have a simple electric bell. This form of electric bell is called a trembling bell, on account of its vibrating armature.
Fig. 117.
| Fig. 118.
|
Fig. 117 shows a form of trembling bell with cover removed. Fig. 118 shows a single-stroke bell, used for fire-alarms and other signal work. In this the armature is attracted but once each time the current passes. As many taps of the bell can be given as desired by pressing the push-button. Fig. 119 shows a gong for railway crossings, signals, etc. Fig. 120 shows a circuit including cell, push-button, and bell, with extra wire for lengthening the line.
Electro-Mechanical Gongs are used to give loud signals for special purposes. The mechanical device is started by the electric current when the armature of the electromagnet is attracted. Springs, weights, etc., are used as the power. Fig. 121 shows a small bell of this kind.
117. Magneto Testing Bells, Fig. 122, are really small hand-power dynamos. The armature is made to revolve between the poles of strong permanent magnets, and it is so wound that it gives a current with a large E. M. F., so that it can ring through the large resistance of a long line to test it.
Magneto Signal Bells, Fig. 123, are used as generator and bell in connection with telephones. The generator, used to ring a bell at a distant station, stands at the bottom of the box. The bell is fastened to the lid, and receives current from a distant bell.
Fig. 121.
| Fig. 122.
|
Fig. 123.
| Fig. 124.
|
118. Electric Buzzers have the same general construction as electric bells; in fact, you will have a buzzer by removing the bell from an ordinary electric bell. Buzzers are used in places where the loud sound of a bell would be objectionable. Fig. 124 shows the usual form of buzzers, the cover being removed.
CHAPTER XVI.
THE TELEPHONE, AND HOW IT TRANSMITS SPEECH.
119. The Telephone is an instrument for reproducing sounds at a distance, and electricity is the agent by which this is generally accomplished. The part spoken to is called the transmitter, and the part which gives sound out again is called the receiver. Sound itself does not pass over the line. While the same apparatus can be used for both transmitter and receiver, they are generally different in construction to get the best results.
Fig. 125.
| Fig. 126.
|
120. The Bell or Magneto-transmitter generates its own current, and is, strictly speaking, a dynamo that is run by the voice. It depends upon induction for its action.
Fig. 125 shows a coil of wire, H, with soft iron core, the ends of the wires being connected to a delicate galvanoscope. If one pole of the magnet H M be suddenly moved up and down near the core, an alternating current will be generated in the coil, the circuit being completed through the galvanoscope. As H M approaches the core the current will flow in one direction, and as H M is withdrawn it will pass in the opposite direction. The combination makes a miniature alternating dynamo.
If we imagine the soft iron core of H, Fig. 125, taken out, and one pole of H M, or preferably that of a bar magnet stuck through the coil, a feeble current will also be produced by moving the soft iron back and forth near the magnet's pole. This is really what is done in the Bell transmitter, soft iron in the shape of a thin disc (D, Fig. 126) being made to vibrate by the voice immediately in front of a coil having a permanent magnet for a core. The disc, or diaphragm, as it is called, is fixed near, but it does not touch, the magnet. It is under a constant strain, being attracted by the magnet, so its slightest movement changes the strength of the magnetic field, causing more or less lines of force to shoot through the turns of the coil and induce a current. The coil consists of many turns of fine, insulated wire. The current generated is an alternating one, and although exceedingly small can force its way through a long length of wire.
Fig. 127 shows a section of a regular transmitter, and Fig. 128 a form of compound magnet frequently used in the transmitter. Fig. 129 shows a transmitter with cords which contain flexible wires.
121. The Receiver, for short lines, may have the same construction as the Bell transmitter. Fig. 130 shows a diagram of two Bell receivers, either being used as the transmitter and the other as the receiver. As the alternating current goes to the distant receiver, it flies through the coil first in one direction and then in the other. This alternately strengthens and weakens the magnetic field near the diaphragm, causing it to vibrate back and forth as the magnet pulls more or less. The receiver diaphragm repeats the vibrations in the transmitter. Nothing but the induced electric current passes over the wires.
122. The Microphone. If a current of electricity be allowed to pass through a circuit like that shown in Fig. 131, which includes a battery, a Bell receiver, and a microphone, any slight sound near the microphone will be greatly magnified in the receiver. The microphone consists of pieces of carbon so fixed that they form loose contacts. Any slight movement of the carbon causes the resistance to the current to be greatly changed. The rapidly varying resistance allows more or less current to pass, the result being that this pulsating current causes the diaphragm to vibrate. The diaphragm has a constantly varying pull upon it when the carbons are in any way disturbed by the voice, or by the ticking of a watch, etc. This principle has been made use of in carbon transmitters, which are made in a large variety of forms.
123. The Carbon Transmitter does not, in itself, generate a current like the magneto-transmitter; it merely produces changes in the strength of a current that flows through it and that comes from some outside source. In Fig. 132, X and Y are two carbon buttons, X being attached to the diaphragm D. Button Y presses gently against X, allowing a little current to pass through the circuit which includes a battery, D C, and a receiver, R. When D is caused to vibrate by the voice, X is made to press more or less against Y, and this allows more or less current to pass through the circuit. This direct undulating current changes the pull upon the diaphragm of R, causing it to vibrate and reproduce the original sounds spoken into the transmitter. In regular lines, of course, a receiver and transmitter are connected at each end, together with bells, etc., for signaling.