Section 40. _electromagnets._

How is a telegram sent?

What carries your voice when you telephone?

So far we have talked about electricity only making heat and light by being forced through something that resists it. But everybody knows that electricity can be made to do another kind of work. It can be made to move things,--to run street cars, to click telegraph instruments, to vibrate the thin metal disk in a telephone receiver, and so on. The following experiments will show you how electricity moves things:

EXPERIMENT 75. Bare an inch of each end of a piece of insulated wire about 10 feet long. Fasten one end to the zinc of your battery or to one wire from the storage battery; wrap the wire around and around an iron machine bolt, leaving the bolt a foot or so from the battery, until you have only about a foot of wire left. Hold your bolt over some iron filings. Is it a magnet? Now touch the free end of your wire to the carbon of your battery or to the other wire from the storage battery, and hold the bolt over the iron filings. Is it a magnet now?

You have completed the circuit by touching the free end of the wire to the free pole of your battery; so the electricity flows through the wire, around the bolt, and back to the battery.

Disconnect one end of the wire from the battery. You have now broken the circuit, and the electricity can no longer flow around the bolt to magnetize it. See if the bolt will pick up the iron filings any more; it may keep a little of its magnetism even when no electricity is flowing, but the magnetism will be noticeably less. When you disconnect the wire so that the electricity can no longer flow through a complete circuit from its source back to its source again, you are said to break the circuit.

EXPERIMENT 76. Examine the cigar-box telegraph (see Appendix B) and notice that it is made on the same principle as was the magnetized bolt in Experiment 75. Complete the circuit through the electromagnet (the bolt wound with wire) by connecting the two ends of the wire that is wrapped around the bolt, with wires from the two poles of the battery. By making and breaking the circuit (connecting and disconnecting one of the wires) you should be able to make the lower bolt jump up and down and give the characteristic click of the telegraph instrument.

In this experiment it does not matter how long the wires are if the batteries are strong enough. Of course it makes no difference where you break the circuit. So you could have the batteries in the laboratory and the cigar box a hundred miles away, with the wire going from the batteries to the bolt and back again. Then if you made and broke the circuit at the laboratory, the instrument would click a hundred miles away. If you want to, you may take the cigar-box telegraph out into the yard, leaving the batteries in the laboratory, while you try to telegraph this short distance.

Examine a regular telegraph instrument. Trace the wire from one binding post, around the coil and through the key, back to the other binding post, and notice how pushing down the key completes the circuit and how raising it up breaks the circuit.

EXPERIMENT 77. Connect two regular telegraph instruments, leaving one at each end of the long laboratory table. Make the connections as follows:

Take a wire long enough to go from one instrument to the other. Fasten the bare ends of this wire into the right-hand binding post of the instrument at your left, and into the left-hand binding post of the instrument at your right; that is, connect the binding posts that are nearest together, as in Figure 141.

Now connect one wire from the laboratory battery to the free post of the right-hand instrument. Connect the other wire from the laboratory battery to the ground through a faucet, radiator, or gas pipe, making the connection firm and being sure that there is a good, clear contact between the bare end of the wire and the metal to which the wire is attached.

Make another ground connection near the left-hand instrument; that is, take a wire long enough to reach from some pipe or radiator to the left-hand telegraph instrument, bind one bare end of this wire firmly to a clean part of the pipe and bring the other end toward the instrument. Before attaching this other end to the free binding post of the left-hand instrument, be sure to open the switch beside the telegraph key by pushing it to your right. Close the switch on the other instrument. Now attach the free ground wire to the free binding post of your telegraph instrument, and press the key. Does the other instrument click? If not, disconnect the ground wire and examine all connections. Also press the sounder of each instrument down and see if it springs back readily. It may be that some screw is too tight, or too loose, or that a spring has come off; tinker awhile and see if you cannot make the instrument work. If you are unable to do so, ask for help.

Figure 141 is a diagram of all the connections.

When you want to telegraph, open the switch of the instrument you want to send from and close the switch of the instrument which is to receive the message.

Holding the key down a little while, then letting it up, makes a "dash," while letting it spring up instantly, makes a "dot."

Practice making dots and dashes. Telegraph the word "cat," using the alphabet shown on the next page. Telegraph your own name; your address.

Here is the Morse telegraph code in dots and dashes:

LETTERS

A B C D E F G ·- -··· ·· · -·· · ·-· --·

H I J K L M N ···· ·· -·-· -·- -- - - -·

O P Q R S T U · · ····· ··-· · ·· ··· - ··-

V W X Y Z & ···- ·-- ·-·· ·· ·· ··· · · ···

NUMERALS

1 2 3 4 5 ·--· ··-·· ···-· ····- ---

6 7 8 9 0 ······ --·· -···· -··- ----

By using the Morse code, telegraph and cable messages are sent all over the world in a few seconds. The ability to send messages in this way arose from the simple discovery that when an electric current passes around a piece of iron, it turns the iron into a magnet.

HOW A TELEPHONE WORKS. A telephone is much like a delicate and complicated telegraph in which the vibrations started by your voice press the "key," and in which the sounder can vibrate swiftly in response to the electric currents passing through the wire. The "key" in the telephone is a thin metal disk that vibrates easily, back of the rubber mouthpiece. Each time an air vibration from your voice presses against it, it increases the current flowing in the circuit. And each time the current in the circuit is increased, the disk in the receiver is pulled down, just as the sounder of a telegraph is pulled down. So every vibration of the disk back of the mouthpiece causes a vibration of the disk in the receiver of the other telephone; this makes the air over it vibrate just as your voice made the mouthpiece vibrate, and you get the same sound.

To make a difference between slight vibrations and larger ones in telephones, there are some carbon granules between the mouthpiece disk and a disk behind it; and there are various other complications, such as the bell-ringing apparatus and the connections in the central office. But the principle of the telephone is almost exactly the same as the principle of the telegraph. Both depend entirely on the fact that an electric current passing around a piece of iron magnetizes the iron.

EXPERIMENT 78. By means of your battery, make an electric bell ring. Examine the bell and trace the current through it. Notice how the current passes around two iron bars and magnetizes them, as it did in the telegraph instrument. Notice that the circuit is completed through a little metal attachment on the base of the clapper, and that when the clapper is pulled toward the electromagnet the circuit is broken. The iron bars are then no longer magnetized. Notice that a spring pulls the clapper back into place as soon as the iron stops attracting it. This completes the circuit again and the clapper is pulled down. That breaks the circuit and the clapper springs back. See how this constant making and breaking of the circuit causes the bell clapper to fly back and forth.

The electric bell, like the telephone and telegraph, works on the simple principle that electricity flowing through a wire that is wrapped around and around a piece of iron will turn that piece of iron into a magnet as long as the electricity flows.

THE ELECTRIC MOTOR. The motor of a street car is a still more complicated carrying out of the same principle. In the next experiment you will see the working of a motor.

EXPERIMENT 79. Connect the wires from the laboratory battery to the two binding posts of the toy motor, and make the motor run. Examine the motor and see that it is made of several electromagnets which keep attracting each other around and around.

Motors, and therefore all things that are moved by electricity, including trolley cars and electric railways, submarines while submerged, electric automobiles, electric sewing machines, electric vacuum cleaners, and electric player-pianos, are moved by magnetizing a piece of iron and letting this pull on another piece of iron. And the iron is magnetized by letting a current of electricity flow around and around it.

The making of various kinds of electromagnets and putting currents of electricity to work is becoming one of the great industries of mankind. Waterfalls are being hitched up to dynamos everywhere, and the water power that once turned the mill wheels now turns millions of coils of wire between the poles of powerful magnets. The current generated in this way is used for all kinds of work--not only for furnishing light to cities, and cooking meals, heating homes, and ironing clothes, but for running powerful motors in factories, for driving interurban trains swiftly across the country, for carrying people back and forth to work in city street cars, for lifting great pieces of iron and steel in the yards where huge electromagnets are used,--for countless pieces of work in all parts of the globe. Yet the use of electricity is still only in its beginning. Tremendous amounts of water power are still running to waste; there is almost no limit to the amount of electricity we shall be able to generate as we use the world's water power to turn our dynamos.

APPLICATION 62. Explain how pressing a telegraph key can make another instrument click hundreds of miles away, and how you can hear over the telephone. Is it vibrations of sound or of electricity that go through the telephone wire, or does your voice travel over it, or does the wire itself vibrate? Explain how electricity can make a car go.

INFERENCE EXERCISE

Explain the following:

371. When a fuse blows out, you can get no light.

372. If you lay your ear on a desk, you hear the sounds in the room clearly.

373. If you touch a live wire with wet hands, you get a much worse shock than if you touch it with dry hands.

374. A park music stand is backed by a sounding board.

375. The clapper of an electric bell is pulled against the bell when you push the button.

376. A hot iron tire put on a wagon wheel fits very tightly when it cools.

377. Candy will cool more rapidly in a tin plate than in a china plate.

378. When a trolley wire breaks and falls to the ground it melts and burns at the point at which it touches the ground.

379. By allowing the electricity from the trolley wire to flow down through an underground coil of wire, a motorman can open a switch in the track.

380. The bare ends of the two wires leading to your electric lamp should never be allowed to touch each other.