Common Science Part 32

352. When you pull the plug out of an electric iron, the iron cools.

353. People who do not hear well sometimes use speaking trumpets.

354. The sounding board of a piano is roughly triangular; the longest strings are the extreme left, and those to the right get shorter and shorter.

355. Birds can sit on live wires without getting a shock.

356. Deaf people can sometimes identify musical selections by holding their hands on the piano.

357. An electric toaster gets hot when a current pa.s.ses through it.

358. The cord of an electric iron sometimes catches fire while the iron is in use, especially if the cord is old.

359. If a live wire touches the earth or anything connected with it, the current rushes into the earth.

360. When you stub your toe, you have to run forward to keep from falling.

SECTION 39. _Short circuits and fuses._

Why does a fuse blow out?

Sometimes during the evening when the lights are all on in your home, some one tinkers with a part of the electric circuit or turns on an electric heater or iron, and suddenly all the lights in that part of the house go out. A fuse has blown out. If you have no extra fuses on hand, it may be necessary to wait till the next day to replace the one that is blown out. It is always a good idea to keep a couple of extra fuses; they cost only 10 cents each. And if you do not happen to know how fuses work or how to replace them when they blow out, it will cost a dollar or so to get an electrician to put in a new fuse. The next three experiments will help you to understand fuses.

[Ill.u.s.tration: FIG. 136. _A_, the "fuse gap" and _B_, the "nail plug."]

EXPERIMENT 72. On the lower wire leading to the electric lamp in the laboratory you will find a "gap," a place where the wire ends in a piece of a knife switch, and then begins again about an inch away in another piece of the switch, as shown in Figure 136. There must be some kind of wire or metal that will conduct electricity across this gap. But the gap is there to prevent as much electricity from flowing through as might flow through copper wire. So never put copper wire across this gap.

If you do, you will have to pay for the other fuses which may blow out. Always keep a piece of fuse wire stretched across the gap. Fuse wire is a soft leadlike wire, which melts as soon as too much electricity pa.s.ses through it.

Unscrew the lamp, and into the socket where it was, screw the plug with the two nails sticking out of it. Turn the electricity on. Does anything happen? Turn the electricity off. Now touch the heads of the two nails together, or connect them with a piece of any metal, and turn on the electricity.

What happens? Examine the pieces of the fuse wire that are left.

It was so easy for the electricity to pa.s.s through the nails and wire, that it gushed through at a tremendous rate. This melted the fuse wire, or blew out the fuse. If the fuse across the gap by the socket had not been the more easily burned out, one or perhaps both of the more expensive fuses up above, where the wire comes in, would have blown out. These cost about 10 cents each to replace, while the fuse wire you burned out costs only a fraction of a cent. If there were no fuses in the laboratory wirings and you had "short circuited" the electricity (given it an easy enough path), it would have blown out the much more expensive fuses where the electricity enters the building. If there were no big fuses where the electricity enters the building, the rush of electricity would make all the copper wires through which it flowed inside the building so hot that they would melt and set fire to the building. As long as you keep a piece of fuse wire across the gap, there is no danger from short circuits.

WHY FUSE WIRE MELTS. For two reasons, the fuse wire melts when ordinary wire would not. First, it has enough resistance to electricity so that if many amperes (much current) flow through, it gets heated. It has not nearly as much resistance, however, as the filament in an electric lamp or even as has the long resistance wire.

It does not become white hot as they do.

Second, it has a low melting point. It melts immediately if you hold a match to it; try this and see. Consequently, long before the fuse wire becomes red hot, it melts in two. It has enough resistance to make it hot as soon as too many amperes flow through; and it has such a low melting point that as soon as it gets hot it melts in two, or blows out. This breaks the circuit, of course, so that no more electricity can flow. In this way the fuse protects houses from catching fire through short circuits.

[Ill.u.s.tration: FIG. 137. What will happen when the pin is thrust through the cords and the electricity turned on?]

Unfortunately, however, the fuse is almost no protection against an electric arc. The copper vapor through which the electricity pa.s.ses in an arc has enough resistance to keep the amperage (current) low; so the arc may not blow out the fuse at all. But if it were not for fuses, there would be about as much danger of houses being set on fire by short circuits as by arcs. Perhaps there would be more danger, because short circuits are the more common.

EXPERIMENT 73. Put a new piece of fuse wire across the fuse gap. Leave the "nail plug" screwed in the socket. Use a piece of flexible lamp cord--the kind that is made of two strands of wire twisted together (see Fig. 137). Fasten one bared end of each wire around each nail of the "nail plug." See that the other ends of the lamp cord are not touching each other.

Turn on the electricity. Does anything happen? Turn off the electricity. Now put a pin straight through the middle of the two wires. Turn on the electricity again. What happens?

There is not much resistance in the pin, and so it allows the electricity to rush through it. People sometimes cause fuses to blow out by pinning pictures to electric lamp wires or by pinning the wires up out of the way.

A SHORT CIRCUIT AN "EASY CIRCUIT." You always get a short circuit when you give electricity an easy way to get from one wire to the other.

But you get no current unless you give it some way to pa.s.s from one wire to the other, thus completing the circuit. Therefore you should always complete the circuit through something which resists the flow of electricity, like an electric lamp, a heater, or an iron. Remember this and you will have the key to an understanding of the practical use of electricity.

The term "short circuit" is a little confusing, in that electricity may have to go a longer way to be short circuited than to pa.s.s through some resistance, such as a lamp. Really a short circuit should be called an "easy circuit" or something like that, to indicate that it is the path of least resistance. Wherever the electricity has a chance to complete its circuit without going through any considerable resistance, no matter how _far_ it goes, we have a short circuit. And since everything resists electricity a little, a large enough flow of electricity would even heat a _copper_ wire red hot; that is why a short circuit would be dangerous if you had no fuses.

_APPLICATION 59._ To test your knowledge of short circuits and fuses, trace the current carefully from the upper wire as it enters the laboratory, through the plug fuse. Show where it comes from to enter the plug fuse, exactly how it goes through the fuse, where it comes out, and where it goes from there.

Trace it on through the cartridge fuse in the same way, through all the switches into the lamp socket, through the lamp, out of the lamp socket to the fuse gap, across this to the other wire, and on out of the room.

It goes on from there through more fuses and back to the dynamo from which the other wire comes.

Test yourself further with the following questions:

1. Where in this circuit is the resistance supposed to be?

2. What happens when you put a good conductor in place of this resistance if the electricity can get from one wire to the other without pa.s.sing through this resistance?

3. Why do we use fuses?

4. What is a short circuit?

5. What makes an electric toaster get hot?

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 pa.s.s 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 pa.s.s through it; _25A_ if it allows 25 amperes to pa.s.s through it, etc. The fuse wire across the fuse gap allows about 8 amperes to pa.s.s 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: