CHAPTER XXVII.

APPLICATIONS OF ELECTRICITY.

_=456. Things Electricity Can Do.=_ Among the almost countless things that electricity can do are the following: It signals without wires. It drills rock, coal, and teeth. It cures diseases and kills criminals. It protects, heats, and ventilates houses. It photographs the bones of the human body. It rings church bells and plays church organs. It lights streets, cars, boats, mines, houses, etc. It pumps water, cooks food, and fans you while eating. It runs all sorts of machinery, elevators, cars, boats, and wagons. It sends messages with the telegraph, telephone, and search-light. It cuts cloth, irons clothes, washes dishes, blackens boots, welds metals, prints books, etc., etc.

As this book deals almost exclusively with experiments, to be performed with simple, home-made apparatus, space cannot be given for a discussion of the many instruments and machines which make electricity a practical every-day thing. (See "Things A Boy Should Know About Electricity.") The principles upon which a few important instruments depend, however, will be given.

=EXPERIMENT 191. To study the action of a simple "telegraph sounder."=

=457. Directions.= (A) Arrange as in Fig. 150. The electromagnet is supported upon its base, as directed in § 407. Coil H, K, and D C are joined in series. The iron strip, I, can be held by the left hand, while K is worked with the right.

(B) Press the key, closing the circuit for different lengths of time, and note that the _armature_, I, responds exactly to the motions at K.

_=458. Discussion.=_ The downward click makes a distinct sound, and in regular instruments the armature is allowed to make an upward click, also. 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 making and connecting a home-made telegraph line, see Apparatus Book.)

_=459. Telegraph Line; Connections.=_ Fig. 151 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, or plan. 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 near each other 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 a wire, R W, or through the earth; but for short lines a return wire is best.

_=460. Operation of Line.=_ Suppose R (right) and L (left) have a line. Fig. 151 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. (See Exp. 191.) As soon as the key is raised, the armatures raise, making the up-click. (See § 458.) As soon as R has finished, he closes his switch, E. L then opens 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 are used for such lines. On large lines the current from a dynamo is used.

=EXPERIMENT 192. To study the action and use of the "relay" on telegraph lines.=

=461. Directions.= (A) Arrange as in Fig. 152. Place K and D C at one end of the table to represent the sending station. At the other end of the table place E, which is the electromagnet of the relay, and H, the electromagnet of the sounder. Connect the ends of E with K and D C, L W being the line wire, and R W the return. In practice, the return is through the earth. The relay armature, R A, should vibrate towards E every time K is pressed. C is a piece of copper against which R A presses each time it is attracted by E, and this closes what is called the local circuit. Connect the poles of another battery, L B, with C and H, and the other end of coil H with R A. The sounder armature, S A, should be arranged as in Exp. 191. Small springs are shown on the two armatures, and these keep them away from the cores when the circuits are open.

(B) Fasten the parts to a board, and study the connections and action of this home-made outfit.

=462. The Relay= replaces the sounder in the line wire circuit, and its coils are usually wound with many turns of fine wire, so that a feeble current will move its nicely adjusted armature. 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. The current passes back from the relay to the sending station through the earth. The relay armature acts as an automatic key to open and close the local circuit, which includes also a battery and sounder. The line current does not enter the sounder. (See "Things A Boy Should Know About Electricity.")

=EXPERIMENT 193. To study the action of a two-pole telegraph instrument.=

=463. Directions.= (A) Arrange as in Fig. 153. Connect the two coils to the connecting plates, as described in § 408. Join a strip of copper Cu with wire 2 leading from D C, and join the zinc of D C to M. The ends of wires 1 and 3 should be near Cu but they must not touch it. If Cu be slightly curved so that its ends are raised above the table, the ends of wires 1 and 3 may be put directly under the ends of Cu; each half of Cu can then be used as a key. Two armatures, A and B, should be held as shown. D C can be placed at one side, of course, its terminals being joined to M and Cu.

(B) Press first one end and then the other of Cu, so that the current will pass through H or E at will.

(C) Paste pieces of paper to the armatures, the left one being marked with a dot, and the other with a dash. The one who sends the message can make dots or dashes at the instrument by pressing the proper key. This form of instrument can be easily made by boys, and the messages are more easily read by the eye than by the ear, as in regular sounders.

=EXPERIMENT 194. To study the action of a simple "single needle telegraph instrument."=

=464. Directions.= (A) Arrange as in Fig. 154. Stick a pin on each side of the N pole of the galvanoscope-needle through the degree-card, so that the needle can make but part of a turn when the circuit is closed.

(B) Touch one lever of the reverser C R, then the other, to see whether connections are right. The needle should be forced against one pin and then against the other. If motions to the left represent _dots_, and those to the right _dashes_, combinations of dots and dashes can be used for letters as in the "sounder" (Exp. 191).

(C) Arrange the apparatus shown in Fig. 122 so that messages can be sent.

=EXPERIMENT 195. To study the action of a simple automatic "contact breaker," or "current interrupter."=

=465. Directions.= (A) Arrange as in Fig. 155. Slip a spring connector attached to wire 1 upon the iron strip I, a short distance from its end. Hold the left-hand end of I firmly in one hand, and with the other hold the connector on wire 2 just above that on 1. The right-hand end of I should be just above the core of H.

(B) Allow the current to pass through the circuit by touching the two connectors together gently. Does the armature make one click, as in the telegraph sounder, or does it vibrate rapidly?

(C) Try the connectors in various positions on I.

_=466. Automatic Current Interrupters=_ are used on bells, buzzers, induction coils, etc. The principle upon which they work is shown in the above experiment (Fig. 155). The current, as it comes from the carbon of D C, is obliged to stop when it reaches I, unless the two connectors touch. As soon as the current passes, I is pulled down and away from the upper connector, and this breaks the circuit. I, being held firmly in the hand, immediately springs back to its former position, closing the circuit. The rapidity of the vibrations depends somewhat upon the position of the connectors upon I. In regular instruments, a platinum point is used where the circuit is broken; this stands the constant sparking at that point.

=EXPERIMENT 196. To study the action of a simple "electric bell," or a "buzzer."=

=467. Directions.= (A) Fig. 156 shows the circuit explained in Exp. 195, with a key or push-button put in, so that the circuit can be closed at a distance from the vibrating armature.

(B) Have a friend work the key while you hold I and wires 1 and 2 as directed in Exp. 195. The circuit must not be broken at two places, of course, so begin by holding the two connectors together. The armature should vibrate rapidly each time K is pressed.

_=468. Electric Bells and Buzzers=_ are very nearly alike in construction; 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.

By placing a bell near the end of the vibrating armature (Fig. 156), so that the bell would be struck by it at each vibration, we should have an electric bell. By making the wires 1 and 3 long, the bell or buzzer can be worked at a distance. (See Apparatus Book, Chapter XV, for Home-made Bells and Buzzers.)

=EXPERIMENT 197. To study the action of a simple telegraph "recorder."=

=469. Directions.= (A) Cut from a tin box, or can, a piece of tin about 4 in. long and 1-1/2 in. wide. Bend this double to make two thicknesses. This will serve as an armature I (Fig. 157). Nail to one end of I a small spool, S, and into this put a short length of lead-pencil, P, which may be held firmly in S by wrapping a little paper around it. Connect the ends of coil H to a key and cell as in Fig. 156.

(B) Hold or fasten I in place, and have a friend make dots and dashes at the key, while you draw a piece of paper past the end of P. A little adjusting will be necessary to get the pencil to write only while the circuit is closed. In regular machines all the parts are automatic.

=EXPERIMENT 198. To study the action of a simple "annunciator."=

=470. Directions.= (A) Arrange as in Fig. 158. Fasten the two electromagnets, H and E, to a board or a piece of stiff cardboard. They may be held in place by passing strings over them and through the board, tying on the other side. The ends of coils H and E should be joined to pieces of tin, A, B, C, by means of connectors. K and K are keys or push-buttons, which in real instruments are in different rooms. Two steel pens may be swung on pins a short distance from the ends of the cores, so that their lower ends will be attracted to the cores the instant the current passes through them. The residual magnetism should hold them against the cores until removed. Hairpins, nails, or needles can be used instead of pens.

(B) Press first one K and then the other to see whether your connections are correct.

_=471. Annunciators.=_ There are many forms of annunciators in use to indicate, in a hotel for example, a certain room when a bell rings at the office. If a bell be included in the circuit between D C and A in Fig. 158, it will ring each time a key is pushed. This will call attention to the fact that some one has rung, and the annunciator will show the location of the special call. Large instruments are made with hundreds of electromagnets, each one answering to a special room. The instrument should be set, of course, after each call. A nail or screw wound with insulated wire can be used for the electromagnets of a home-made annunciator.

=EXPERIMENT 199. To study the shocking effects of the "extra current."=

=472. Directions.= (A) Use the two electromagnets joined to the connecting plates (Fig. 132), to generate a self-induced or extra current. Connect R of Fig. 132 with the zinc of a dry cell, and between L and the carbon of the cell place a key; in other words, join the electromagnets, cell, and key in series. Two good cells in series can be used to advantage.

(B) Wet the ends of two fingers of the left hand, press one upon L and the other on R, thus making a shunt with your hand. With the right hand work the key rapidly. If the current is strong enough you should feel a slight shock in the fingers each time the circuit is broken. The extra current (§ 444) causes the shock as it shoots through the fingers.

(C) If you have electric bells or telegraph sounders use them for this experiment.

_=473. Induction Coils=_ are instruments for producing induced currents of high E. M. F. The apparatus shown in Fig. 141 forms a simple induction coil. The _primary_ coil is made of coarser wire and has less turns of wire than the _secondary_ coil. The current in the primary circuit is usually interrupted by an _automatic interrupter_ (Exp. 195), thus producing an alternating current in the secondary coil, the voltage of which depends upon the relative number of turns in the two coils. Induction coils are used in telephone work, for medical purposes, for X-ray work, etc., etc.

(For Home-made Induction Coils see Apparatus Book, Chapter XI.)

=474. Action of Induction Coils.= Fig. 159 shows a top view of one of the home-made induction coils described, in full, in the Apparatus Book. Wires 5 and 6 are the ends of the primary coil, while wires 7 and 8 are the terminals of the secondary coil. The battery wires should be joined to binding-posts W and X, and the handles to Y and Z. Fig. 160 shows the details of the automatic interrupter which is placed in the primary circuit.

If the current 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 S I, as A is no longer attracted; the circuit being closed, the operation is rapidly repeated. (For commercial forms and uses of induction coils see "Things A Boy Should Know About Electricity.")

_=475. Transformers=_, like induction coils, are instruments for changing the E. M. F. and strength of currents. There is very little loss of energy in well-made transformers. They consist of two coils of wire on the same core; in fact, an induction coil may be considered a transformer. If the secondary coil has 100 times as many turns of wire as the primary, a current with an E. M. F. of 100 volts can be taken from the secondary coil, when the E. M. F. of the current passing through the primary is 1 volt; but the _strength_ (amperes) of the secondary current will be but one-hundredth that of the primary current. By using the coil of fine wire as the primary, the E. M. F. of the current that comes from the other coil will be but one-hundredth that in the fine coil. It will have 100 times its strength, however. Continuous currents from cells or dynamos must be interrupted, as in induction coils, to be transformed from one E. M. F. to another. Transformers are now largely used in lighting and power circuits, etc. (See "Things A Boy Should Know About Electricity.")

_=476. The Dynamo.=_ We saw in the Exps. of Chapter XXV. that currents of electricity can be generated in a coil of wire (closed circuit) by rapidly moving it through the field of a magnet. As shown by the experiments, this can be accomplished in many ways. The dynamo is a machine for doing this on a large scale, the coils being given a rotary motion in a very strong magnetic field; and as the number of lines of force that cut the coil is constantly changing, there is a current in the coil as long as power is applied, and this current is led from the machine by proper devices.

_The dynamo is a machine for converting mechanical energy into an electric current, through electromagnetic induction._

If a loop of wire (Fig. 161) be so arranged on bearings at its ends that it can be made to revolve, a current will flow through it in one direction during one-half of the revolution, and in the opposite direction during the other half, it being insulated from all external conductors. Such a current inside of the machine would be of no value; it must be led out to external conductors. Some sort of sliding contact is necessary to connect a revolving conductor with a stationary one.

Fig. 162 shows the ends of a coil joined to two rings, X, Y, which are insulated from each other, and which rotate with the coil. Two stationary pieces of carbon, A, B, called _brushes_, press against the rings, and to these are joined wires which complete the circuit, and which lead out where the current can do work. The arrows show the direction of the current during one-half of a revolution. The rings form a _collector_, and this arrangement gives an alternating current.

In Fig. 163 the ends of the coil are joined to the two halves of a cylinder. These halves, X and Y, are insulated from each other and from the axis. The current flows from X onto the brush A, through some external circuit where it does work, and thence back through brush B onto Y. By the time that Y gets around to A the direction of the current in the loop has reversed, so that it passes towards Y; but it still enters the outside circuit through A because Y is then in contact with A. This device is called a _commutator_, and it allows a constant or direct current to leave the machine.

In regular machines there are many loops of wire and several segments to the commutator. The rotating coils are wound upon an iron core, so that the lines of force, in passing from one pole to the other, will meet with as little resistance as possible. The coils, core, and commutator, taken together, are called the _armature_. The magnets which furnish the field are called the _field-magnets_. These are electromagnets, the current from the dynamo, or a part of it, being used to excite them. There are many forms of dynamos, and many ways of winding the armature and field-magnets, but space will not permit a discussion of them here. (See "Things a Boy Should Know About Electricity.")

_=477. The Electric Motor.=_ Experiments have shown that motion can be produced by the electric current in many ways. The galvanoscope may be considered a tiny motor.

_An electric motor is a machine for transforming electric energy into mechanical power._

While the electric motor is similar in construction to the dynamo, it is opposite to it in action. Motors receive current and produce motion. The motion is a rotary one, the power being applied to other machines by means of belts or gears.

=EXPERIMENT 200. To study the action of the telephone.=

=478. Directions.= (A) Join the ends of coil H (Fig. 164) to the astatic galvanoscope. Move magnet M back and forth in front of the soft iron core, while H is held in position. Watch the needle. Imagine that vibrations in the air caused by the voice are strong enough to give M a slight motion to and fro, and you can see how a current would be sent through the galvanoscope by speaking against M.

_=479. 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 the sound out again is called the _receiver_. Sound itself does not pass over the line. Although the same apparatus may be used for both transmitter and receiver, they are generally different in construction.

_=480. The Bell or Magneto-transmitter=_ generates its own current, and is, strictly speaking, a dynamo that is run by the voice. You have seen, by experiments, that a current can be generated in a coil of wire by moving a magnet back and forth in front of its soft iron core. In the telephone this process is reversed, soft iron in the shape of a thin disc (D, Fig. 165) being made to vibrate by the voice immediately in front of a coil having a permanent magnet, M, for a core.

The soft iron diaphragm is fixed near, but it does not touch the magnet. The coil consists of many turns of fine insulated wire. The current generated is an alternating one and exceedingly feeble; in fact, it can not be detected by a galvanoscope.

_=481. The Receiver=_ has the same construction as the bell transmitter, and receives the currents from the line. As the diaphragm is always attracted by the magnet, it is under a constant strain. This strain is increased when a current passes through the coil in a direction that adds strength to the magnet, and decreased when the current weakens the magnet.

When the current through the coil is always in the same direction, but varies in strength, the diaphragm will vibrate on account of the varying pull upon it.

When the current through the coil is an alternating one, the same result is obtained, as the magnet gets weaker and stronger many times per minute. Fig. 166 shows two bell instruments joined, either being used as the transmitter and the other as the receiver.

_=482. 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. 167, X and Y are two carbon buttons, X being attached to the diaphragm, D. Button Y presses gently against X. 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, in which also is the receiver, R. 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.

_=483. Induction Coils in Telephone Work.=_ As the resistance of telephone lines is large, a current with a fairly high E. M. F. is desired. While the current from one or two cells is sufficient to work the transmitter, it is not strong enough to force its way over a long line. To get around this difficulty an induction coil is used to transform the battery current, that flows through the transmitter and primary coil, into a current with a high E. M. F. that can go into the main line and force its way to a distant receiver.

The battery current in the primary coil is undulating, but always in the same direction, the magnetic field around the core getting weaker and stronger. This causes an alternating current in the secondary coil and main line.

Fig. 168 shows the two coils, P, S, of the induction coil. The primary, P, is joined in series with a cell and transmitter. The secondary coil, S, is joined to the receiver. One end of S can be grounded, the current completing the circuit through the earth and into the receiver through another wire entering the earth. There are many forms of transmitters. (See "Things a Boy Should Know About Electricity.")

_=484. Electric Lighting and Heating.=_ Whenever resistance is offered to the electric current, heat is produced. By proper appliances, the heat of resistance can be applied just where it is needed, and many commercial processes depend upon electricity for their success. Dynamos are used to generate currents for lighting and heating purposes. There are two great systems of lighting, the one by _arc_ lamps and the other by _incandescent_ lamps. (See "Things a Boy Should Know About Electricity.")

_=485. Arc Lamps=_ produce a light when a current passes from one carbon rod to the other across an air-space. As the current starts through the lamp, the ends of the carbons touch, and the imperfect contact causes resistance enough to heat the ends red-hot. They are then automatically separated, and the current passes from one to the other, causing the "arc." The resistance of the air-space is reduced by the intensely heated vapor and flying particles of carbon.

_=486. The Incandescent Lamp=_ consists of a glass bulb, in which is a vacuum, and the light is caused by the passage of a current through a thin fibre of vegetable carbon, enclosed in the vacuum. The fibre would burn instantly if allowed to come in contact with the air. The fibres have a high resistance, and are easily heated to incandescence.