361. If the insulation wears off both wires of a lamp cord, the fuse will blow out.
362. Street cars are heated by electricity.
363. The handles of pancake turners are often made of wood.
364. Glue soaks into the pores of pieces of wood and gradually hardens.
365. The glue then holds the pieces tightly together.
366. You need a fuse of higher amperage, as a 10-ampere fuse, instead of a 6-ampere one, where you use electricity for an iron, and one of still higher amperage for an electric stove.
367. You should be careful about turning on electric lights or doing anything with electric wires when you are on a cement, iron, or earthen floor, or if you are standing in water.
368. The keys and b.u.t.tons with which you turn on electric lights are usually made of a rubber composition.
369. Defective wiring, because of which bare wires may touch, has caused many fires.
370. A person wearing gla.s.ses can sometimes see in them the image of a person behind him.
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:
[Ill.u.s.tration: FIG. 138. The magnetized bolt picks up the iron filings.]
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_.
[Ill.u.s.tration: FIG. 139. Sending a message with a cigar-box telegraph.]
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.
[Ill.u.s.tration: FIG. 140. Connecting up a real telegraph instrument.]
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.
[Ill.u.s.tration: FIG. 141. Diagram showing how to connect up two telegraph instruments. The circles on the tables represent the binding posts of the instruments.]
[Ill.u.s.tration: FIG. 142. Telegraphing across the room.]
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 pa.s.ses 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 pa.s.sing 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 pa.s.sing 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 pa.s.ses 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.
[Ill.u.s.tration: FIG. 143. The bell is rung by electromagnets.]