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CHAPTER XXV

HOW MAN USES ELECTRICITY

230. What is an Electromagnet? — Have you ever seen men handle a great heap or carload of scrap iron ? Such iron is valuable because it can be used in making steel;

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(Courtesy of the American Hoist & Derrick Co.) Fig. 148. - The magnetic crane has a huge, powerful electromagnet that picks up scrap iron, pig iron, and other iron and steel objects, and drops them wherever they are wanted.

but it is made up of pieces of all shapes and sizes, and is very hard to handle. Do you know the modern way of loading or unloading scrap iron? Instead of picking it up, piece by piece, men use a great steel arm, called a crane, at the end of which there is a powerful magnet. The pieces of iron, big and little, are attracted by the magnet and lifted out of the heap. All sorts of iron and steel objects, if not too large, can be handled in the same way (Fig. 148).

You can see readily that we might get a horseshoe or bar magnet large enough to pick up the iron or steel object; but how can we make the magnet drop its load at the right time and place? We must have some way of causing the magnet to lose its magnetism instantly. We can do this if we use an electromagnet (Fig. 149).

You will remember that when a metal wire is carrying a current Fig. 149. — An electromagnet made of a bar of

soft iron surrounded by a coil of insulated wire of electricity, and carrying a current. it is held over a compass, it causes the compass needle to change its position. If the wire is carrying a strong enough current, it will pick up iron or steel filings just like a magnet. The reason for this behavior is that a wire carrying a current is surrounded by a magnetic field. What happened when you put an unmagnetized needle inside a coil of wire and passed a current through the wire? The needle became magnetized. What do you think would happen if you were to put a piece of soft iron, such as a thick wire, or an iron bolt, inside a coil of wire through which a current is passing? Try it. The soft iron becomes a temporary magnet (what does this mean?), just as if it were in the field of a bar magnet (see § 217). The instant the magi netic field is removed, the soft iron ceases to be a magnet.

So it happens that if we want the magnetic crane to pick up an iron or steel object, we turn on the switch that makes the electric circuit (sŭr'kỉt) complete and allows the current to flow through the coil of the electromagnet. When we want the load to be dropped, we break the circuit; the soft-iron core then ceases to be a magnet, and gravity pulls the load down.

The wire of the coils must be covered, or insulated; for if one turn of wire touches another, the current crosses over by a short cut instead of going through the full length of the wire. This is called “short circuiting” the current. It is important to have a large number of turns in the coil; for the more of them there are, the stronger the electromagnet which can be made. An interesting fact has been found to be true of the soft-iron core; a bundle of small wires makes a much stronger magnet than a single piece of iron having the size of the bundle.

Could we use an electromagnet to sweep up the iron chips and turnings of a factory?

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231. How Do We Ring an Electric Bell? — Have you ever seen one of the old-fashioned house bells which you

rang by pulling a knob? There was a wire attached to the knob, and there was a spring attached to the other end of the wire. When you pulled the knob, you set the spring in motion, and the moving spring jingled the bell. The modern doorbell can be put in almost any out-of-the-way place, and its wire can go around corners and through walls. One thing it must have, however, which the older bell did not need: a battery, or

H some other source of an electric current.

Examine an electric bell, and compare it with the diagram in Fig. 150.

Electromagnet

Spring

Push Button

Trace the circuit from the battery to the push button. Here the circuit is broken, and no current can pass to the electromagnet. But if we press the push button against the little metal triangle shown below it, we close the circuit. Then the cur

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: rent passes through the coil, and causes the core to become a magnet. The core now draws to itself the piece of iron (the arma- Fig. 150. The electric ture) attached to the hammer of the bell. bell is rung by an electroThe hammer is drawn in that way also, and hammer to the bell when

magnet which draws the strikes the bell. As a result we hear a the circuit is complete. The sound.

spring throws the hammer

back as soon as the circuit At this point another interesting thing is broken. happens. When the armature is drawn forward, what becomes of the circuit? You will see that it must be broken at the place at which the armature touched the metal stop. If the circuit is broken, what must happen to the electromagnet? Of course it will lose its power to attract the armature. The metal below the armature is a spring; as soon as the magnet ceases to pull the armature, the spring throws the armature back against the stop. If you are still pressing the push button, the circuit will be closed once more, and the whole performance will be repeated.

Thus the ringing of the doorbell is a series of rapid sounds: the hammer is thrown against the bell, withdrawn, and thrown back again, many times every second of time.

What will be the effect if the armature or the core becomes rusty or covered with grease and dust? If the spring becomes too weak to throw the armature back against its stop?

232. How Can a Telegraph Wire Carry a Message ? Think what a convenience the telegraph is. If the wires are working properly, we can get a message to a distant city, and have the answer back again before we could write a letter, to say nothing of getting it delivered.

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Fig. 151. The electric telegraph is an electromagnet which pulls down the sounder when the key is pressed. When the key is released, the spring pulls the sounder up.

If possible, examine a key and sounder instrument, and compare it with the diagram of Fig. 151, so as to learn how the telegraph works. In the doorbell we have a wire connection both ways, but in the telegraph the “return

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