6. Why should you not stick pins through electric cords?
Experiment 74. Take the fuse wire out of the fuse gap and put a single strand of zinc shaving in its place. Instead of the nail plug, screw the lamp into the socket. Do not turn on the switch that lets the electricity flow through the resistance wire, but turn on the electricity so that the lamp will glow. Does the zinc shaving work satisfactorily as a fuse wire? Now turn the electricity on through the resistance wire. What happens?
When are the greater number of amperes of electricity flowing through the zinc shaving? (Note. "Amperes" means the amount of current flowing.) Can the zinc shaving stand as many amperes as the fuse wire you ordinarily use? Which lets more electricity pass through it, the lamp or the resistance wire? Why do electric irons and toasters often blow out fuses? If this happens at your home, examine the fuse and see how many amperes (how much current) it will allow to flow through it. It will say 6A if it allows 6 amperes to pass through it; 25A if it allows 25 amperes to pass through it, etc. The fuse wire across the fuse gap allows about 8 amperes to pass through before it melts. The zinc shaving allows only about 2. Read the marks on the cartridge and plug fuses. How many amperes will they stand?
Application 60. A family had just secured an electric heater. The first night it was used, the fuse blew out.
The boy said: "Let's put a piece of copper wire across the fuse socket; then there can't be any more trouble."
The father said that they had better get a new fuse to replace the old one. The old fuse was marked 10A.
Was the boy or was the father right? If the father was right, should they have got a fuse marked 6A, one marked 10A, or one marked 15A?
Application 61. The family were putting up an extension light. They wanted the cord held firmly up out of the way. One suggested that they drive a nail through both parts of the cord and into the wall. Another thought it would be better to put a loop of string around the cord and fasten the loop to the wall. A third suggested the use of a double-pointed carpet tack that would go across the wires, but not through them, and if driven tightly into the wall would hold the wire more firmly than would the loop.
Which way was best?
Inference Exercise
Explain the following:
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 buttons 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 glasses 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:
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.