General Science Part 26

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.

An electric current can separate a liquid into some of its various const.i.tuents and to deposit one of the metal const.i.tuents on the negative electrode.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 the thickness 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 const.i.tuent 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.

CHAPTER x.x.xII

MODERN ELECTRICAL INVENTIONS

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 cl.u.s.ter 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.

[Ill.u.s.tration: FIG. 210.--A wire carrying current attracts iron filings.]

[Ill.u.s.tration: 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 cl.u.s.ters to the ends of 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: FIG. 213.--An electromagnet.]

[Ill.u.s.tration: 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 b.u.t.ton (Fig. 215) connection is made with a battery, and current flows through the wire wound on the iron spools, and further to the screw _P_ which presses against the soft iron strip or armature _S_; and from _S_ the current 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, _S_ no longer touches the screw _P_, 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 at _P_ through 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 at _P_ and the entire operation is repeated. So long as we press the b.u.t.ton this process continues producing what sounds like a continuous jingle; in reality the clapper strikes the bell every time a current pa.s.ses through the electromagnet.

[Ill.u.s.tration: FIG. 215.--The electric bell.]

298. The Push b.u.t.ton. The push b.u.t.ton 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 b.u.t.ton 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 b.u.t.ton is pressed, the current has a complete path, provided there is the proper connection at _S_. That is, the pressure on the push b.u.t.ton permits current to flow to the bell. The flow of this current then depends solely upon the connection at _S_, which is alternately made and broken, and in this way produces sound.

[Ill.u.s.tration: FIG. 216.--Push b.u.t.ton.]

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 b.u.t.ton 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 pa.s.s through air. Many substances besides air stop the flow of electricity. If a strip of gla.s.s, 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 gla.s.s k.n.o.bs 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 b.u.t.ton, 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.

[Ill.u.s.tration: FIG. 217.--Diagram of the electric telegraph.]

The fundamental connections of the telegraph are shown in Figure 217.

If the key _K_ is 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, pa.s.ses through the electromagnet _M_, and thence back to Philadelphia. As long as the key _K_ is pressed down, the coil _M_ acts as a magnet and attracts and holds fast the armature _A_; but as soon as _K_ is released, the current is broken, _M_ loses its magnetism, and the armature is pulled back by the spring _D_. By a mechanical device, tape is drawn uniformly under the light marker _P_ attached to the armature. If _K_ is 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. If _K_ is closed for a longer time, a short dash is made by the marker, and, in general, the length of time that _K_ is 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 s.p.a.ces, is given in Figure 218.

[Ill.u.s.tration:

A .- H .... O . . U ..- B -... I .. P ..... V ...- C .. . J -.-. Q ..-. W .-- D -.. K -.- R . .. X .-.. E . L --- S ... Y .. .. F .-. M - - T - Z ... . G --. N -.

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.

[Ill.u.s.tration: 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 be imitated by the motion and the resulting clicks of the sounder. By means of these varying clicks of the sounder, the operator interprets the message.

[Ill.u.s.tration: 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 pa.s.ses 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 the _relay_. 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 strong sounder. For example, the relay is so arranged that current from the main line runs through it exactly as it runs through _M_ in 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 subst.i.tuted 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 a single wire in one second.