Common Science Part 28

Now suppose you bring one loop of the long wire out, as shown in Figure 115, and suppose you spin the rest of the loops between the poles of the magnet. Then, to flow through the loops by the magnet the electricity will have to go clear out through the long loop and back again. While it is flowing through this long loop, we can make it work. We can cut the long loop and attach one broken end to one part of an electric lamp and the other end to the other part, so that the electricity has to flow through the lamp in order to get back to the spinning coil of wire, as shown in Figure 116. Such an arrangement as this is really an extremely simple dynamo.

[Ill.u.s.tration: FIG. 116. If the electricity pa.s.ses through a lamp on its way around the circuit the filament of the lamp glows.]

[Ill.u.s.tration: FIG. 117. A dynamo in an electric light plant.]

You could make a dynamo that would actually work, by arranging such an apparatus so that the coil would spin between the poles of the magnet. But of course the big commercial dynamos are very much more complicated in their construction. Figure 116 shows only the general principle on which they work. The main point to note is that by spinning a coil of wire between the poles of a magnet, you can make electricity flow rapidly through the wire. And it is in this way that most of the electricity we use is made.

The power spinning the coil of wire is sometimes steam, and sometimes gasoline or distillate; and water power is very often used. A large amount of our electricity comes from places where there are waterfalls. Niagara, for instance, turns great dynamos and generates an enormous amount of electricity.

WHY MANY AUTOMOBILES HAVE TO BE CRANKED. In an automobile, the magneto is a little dynamo that makes the sparks which explode the gasoline.

While the automobile is going the engine spins the coil of wire between the magnets, but at starting you have to spin the coil yourself; and doing that is called "cranking" the automobile.

"Self-starters" have a battery and motor to spin the coil for you until the engine begins to go; then the engine turns the coil of the magneto.

HOW OLD-FAs.h.i.+ONED TELEPHONES ARE RUNG. The old-fas.h.i.+oned telephones, still often used in the country, have little cranks that you turn to ring for central. The crank turns a coil of wire between the poles of the magnet and generates the electricity for ringing the bell.

These little dynamos, like those in automobiles, are usually called magnetos.

[Ill.u.s.tration: FIG. 118. The magneto in an automobile is a small dynamo.]

ALTERNATING CURRENT. For the sake of simplicity and convenience we speak of electricity as always flowing in through one wire and out through the other. With batteries this is actually the case. It is also the case where people have what is called _direct-current_ (d.

c.) electricity. But it is easier to raise and lower the voltage (pressure) of the current if instead of being direct it is _alternating_; that is, if for one instant the electricity flows in through one wire and out through the other, the next instant flowing the opposite way, then the first way again, and so on. This kind of current is called _alternating current_ (a. c.), because the current alternates, coming in the upper wire and out of the lower for a fraction of a second; then coming in the lower and out of the upper for the next fraction of a second; then coming in the upper again and out of the lower for a fraction of a second; and so on, back and forth, all the time. For heating and lighting, this alternating current is just as good as the direct current, and it is probably what you have in your own home. For charging storage batteries and making electromagnets, separating water into two gases, and for running certain kinds of motors, however, the direct current is necessary.

Find out whether the current in your laboratory is direct or alternating.

_APPLICATION 49._ Explain why we need fuel or water to generate large currents of electricity; how we can get small amounts of electricity to flow without using dynamos; why automobiles must be cranked unless they have batteries to start them.

INFERENCE EXERCISE

Explain the following:

301. Mexican water jars are made of porous clay; the water that seeps through keeps the water inside cool.

302. When you crank an automobile, electricity is generated.

303. Potatoes will not cook any more quickly in water that is boiling violently than in water that is boiling gently.

304. When you brush your hair on a winter morning, it sometimes stands up and flies apart more and more as you continue to brush it.

305. You cannot see a person clearly through a ground-gla.s.s window, although it lets most of the light through.

306. There is a layer of coa.r.s.e, _light-colored_ gravel over the tar on roofs, to keep the tar from melting.

307. It is very easy to slip on a well-waxed hardwood floor.

308. If you have a silver filling in one of your teeth and you touch the filling with a fork or spoon, you get a slight shock.

309. You can shake a thing down into a bottle when it will not slip down by itself.

310. If you rub a needle across one pole of a magnet three or four times in the same direction, then float it on a cork in water one end of the needle will point north.

SECTION 34. _Conduction of electricity._

How does electricity travel?

Why do you get a shock if your hands are wet when you touch a live wire?

If you were to use a piece of string instead of a copper wire to go from one pole of a battery to another or to spin between the poles of the magnet of the dynamo, you could get no flow of electricity to speak of. Electrons do not flow through string easily, but they flow through a copper wire very easily. Anything that carries, or conducts, electricity well is called a _good conductor_. Anything that carries it poorly is called a _poor conductor_. Anything that allows practically no electricity to pa.s.s through it is called an _insulator_.

EXPERIMENT 65.[5] Turn on an electric lamp. Turn it off by opening the knife switch. Cover the blade of the knife switch with a fold of paper and close it. Will the lamp glow? Try a fold of dry cloth; a fold of the same cloth wet. Connect the blade to the slot with a piece of iron; with a piece of gla.s.s; with porcelain; with rubber; with dry wood; with wood that is soaking wet; with a coin. Which of these are good conductors of electricity? Which could be used as insulators?

[Footnote 5: Read footnote, page 226, before doing this experiment.]

[Ill.u.s.tration: FIG. 119. Electricity flows through the coin.]

HOW YOU CAN GET AN ELECTRICAL SHOCK. A person's body is not a very good conductor of electricity, but will conduct it somewhat. When electricity goes through your body, you get a shock. The shock from the ordinary current of electricity, 110 volts, is not enough to injure you at all; in fact, if you were standing on dry wood, it would be _safe_, although you would get a slight shock, to connect the blade of a knife switch to the slot of the switch, through your hand or body. Your body would not allow enough current to pa.s.s through it to light the lamp. Stronger currents, like those of power lines and even trolley wires, are extremely dangerous.

All the electric wires entering your house are made of copper. They are all covered with cloth and rubber and are fastened with gla.s.s or porcelain k.n.o.bs. The reason is simple: Copper and practically all other metals are very good conductors of electricity; that is, they allow electricity to pa.s.s through them very easily. Cloth, rubber, gla.s.s, and porcelain are very poor conductors, and they are therefore used as insulators,--to keep the electricity from going where you do not want it to go.

[Ill.u.s.tration: FIG. 120. Will electricity go through the gla.s.s?]

EXPERIMENT 66. To each binding post of an electric bell fasten a piece of insulated copper wire with bare ends and at least 4 feet long. Connect the free end of one of these wires with one pole of a battery, using a regular laboratory battery or one you made yourself. Attach one end of another piece of wire a foot or so long, with bare ends, to the other pole of the battery. Touch the free end of this short wire to the free end of the long wire, as shown in Figure 120. Does the bell ring?

If it does not, something is wrong with the connection or with the battery; fix them so that the bell will ring. Now leave a gap of about an inch between the free end of the long wire and the free end of the short wire. Try making the electricity flow from the short wire into the long one through a number of different things, such as string, a key, a knife, a piece of gla.s.s tubing, wet cloth, dry cloth, rubber, paper, a nail, a dish of mercury (dip the ends of the wire into the dish so that they both touch the mercury at the same time), a dish of water, a stone, a pail, a pin, and anything else that you may like to try.

[Ill.u.s.tration: FIG. 121. Electrical apparatus: _A_, plug fuse; _B_, cartridge fuse; _C_, knife switch; _D_, snap switch; _E_, socket with nail plug in it; _F_, fuse gap; _G_, flush switch; _H_, lamp socket; _I_, _J_, _K_, resistance wire.]

Each thing that makes the bell ring is a good conductor. Each one that will not make it ring is a poor conductor or an insulator. Make a list of the things you have tried; in one column note the good conductors, and in another column note the insulators and poor conductors.

The water and wet cloth did not ring the bell, but this is because the pressure or voltage of the electricity in the batteries is not very high. In dealing with high-power wires it is much safer to consider water, or anything wet, as a pretty good conductor of electricity.

Absolutely pure, distilled water is an extremely poor conductor; but most water has enough minerals dissolved in it to make it conduct electricity fairly well. In your list you had better put water and wet things in the column with the good conductors.

_APPLICATION 50._ Robbers had cut the telegraph line between two railroad stations (Fig. 122). The broken ends of the wire fell to the ground, a number of feet apart. A farmer caught sight of the men speeding away in an automobile and he saw the cut wires on the ground. He guessed that they had some evil purpose and decided to repair the damage. He could not bring the two ends of the wire together. He ran to his barn and found the following things there:

A ball of cord, a pickax, a crowbar, some harness, a wooden wagon tongue, a whip, a piece of iron wire around a bale of hay (the wire was not long enough to stretch the whole distance between the two ends of the telegraph wire, even if you think he might have used it to patch the gap), a barrel with four iron hoops, and a rope.

Which of these things could he have made use of in connecting the broken ends of the telegraph wire?

[Ill.u.s.tration: FIG. 122. Which should he choose to connect the broken wires?]

_APPLICATION 51._ A man was about to put in a new socket for an electric lamp in his home. He did not want to turn off the current for the whole house, as it was night and there was no gas to furnish light while he worked.

"I've heard that if you keep your hands wet while you work, the film of water on them will keep you from getting a shock," his wife said.

"Don't you wet your hands, Father," said his 12-year-old boy; "keep them dry, and handle the wires with your pliers, so that you won't have to touch it."

"I advise you to put on your canvas gloves while you work; then you can't get a shock," added another member of the family.

"That's a good idea," said the wife, "but wet the gloves, then you will have the double protection of the water and the cloth."

The man laughed and went to work on the socket. He did not get a shock. Which advice, if any, do you think he followed?