Chapter Eight

ELECTRICITY

SECTION 33. Making electricity flow.

What causes a battery to produce electricity?

What makes electricity come into our houses?

The kind of electricity you get from rubbing (friction) is not of much practical use, you remember. Men had to find a way to get a steady current of electricity before they could make electricity do any work for them. The difference between static electricity--when it leaps from one thing to another--and flowing electricity is a good deal like the difference between a short shower of rain and a river. Both rain and river are water, and the water of each is moving from one place to another; but you cannot get the raindrops to make any really practical machine go, while the rivers can do real work by turning the wheels in factories and mills.

Within the past century two devices for making electricity flow and do work have been perfected: One of these is the electric battery; the other is the dynamo.

THE ELECTRIC BATTERY. A battery consists of two pieces of different kinds of metal, or a metal and some carbon, in a chemical solution. If you hang a piece of zinc and a carbon, such as comes from an arc light, in some water, and then dissolve sal ammoniac in the water, you will have a battery. Some of the molecules of the sal ammoniac divide into two parts when the sal ammoniac gets into the water, and the molecules continue to divide as long as the battery is in use or until it "wears out." One part of each molecule has an unusually large number of electrons; the other part has unusually few. The parts with unusually large numbers of electrons gather around the zinc; so the zinc is negatively charged,--it has more than the ordinary number of electrons. The part of the sal ammoniac with unusually few electrons goes over to the carbon; so the carbon is positively charged,--it has fewer than the ordinary number of electrons.

MAKING THE CURRENT FLOW. Now if we can make some kind of bridge between the carbon and the zinc, the electrons will flow from the place where there are many to the place where there are few. Electrons can flow through copper wire very easily. So if we fasten one end of the copper wire to the carbon and the other end to the zinc, the electrons will flow from the zinc to the carbon as long as there are more electrons on the zinc; that is, until the battery wears out. Therefore we have a steady flow of electricity through the wire. While the electricity is flowing from one pole to the other, we can make it do work.

EXPERIMENT 64. Set up two or three Samson cells. They consist of a glass jar, an open zinc cylinder, and a smaller carbon cylinder. Dissolve a little over half a cup of sal ammoniac in water and put it into the glass jar; then fill the jar with water up to the line that is marked on it. Put the carbon and zinc which are attached to the black jar cover into the jar. Be careful not to let the carbon touch the zinc. One of these cells will probably not be strong enough to ring a doorbell for you; so connect two or three together in series as follows:

Fasten a piece of copper wire from the carbon of the first cell to the zinc of the second. If you have three cells, fasten another piece of wire from the carbon of the second cell to the zinc of the third, as shown in Figure 111.

Fasten one end of a copper wire to the zinc of the first cell and the other end of this wire to one binding post of an electric bell. Fasten one end of another piece of copper wire to the carbon of the third cell, if you have three, and touch the other end of this wire to the free binding post of the electric bell. If you have everything connected rightly, the bell should ring.

DIFFERENT KINDS OF BATTERIES. There are many different kinds of batteries. The one you have just made is a simple one frequently used for doorbells. Other batteries are more complicated. Some are made with copper and zinc in a solution of copper sulfate; some, even, are made by letting electricity from a dynamo run through a solution from one lead plate to another until a chemical substance is stored on one of them; then, when the two lead plates are connected by a wire, the electrons run from one to the other. This kind of battery is called a storage battery, and it is much used in submarines and automobiles.

But all the different batteries work on the same general principle: A chemical solution divides into two parts, one with many electrons and the other with a less number. One part of the solution gathers on one pole (piece of metal in the solution) and charges it positively; the other part gathers on the other pole and charges it negatively. Then the electricity flows from one pole to the other.

POSITIVE AND NEGATIVE POLES. Before people knew anything about electrons, they knew that electricity flowed from one pole of a battery to the other. But they always said that it flowed from the carbon to the zinc; and they called the carbon the positive pole and the zinc the negative. Although we now know that the electrons flow from the zinc to the carbon, it is much more convenient to use the old way of speaking, as was explained on page 199. Practically, it makes no difference which way the electrons are going as long as a current of electricity is flowing through the wire from one pole of the battery to the other pole. So every one speaks of electricity as flowing from the positive pole of a battery (usually the carbon or copper) to the negative pole (usually the zinc), although the electrons actually move in the other direction.

Batteries make enough electricity flow to do a good deal of work. But they are rather expensive, and it takes a great many to give a flow of electricity sufficient for really heavy work, such as running street cars or lighting a city. Fortunately there is another way of getting large amounts of electricity to flow. This is by means of dynamos.

HOW A DYNAMO MAKES A CURRENT FLOW. To understand a dynamo, you must first realize that there are countless electrons in the world--perhaps all things are made entirely of them. But you remember that when we want to get these electrons to do work we must make them flow. This can be done by spinning a loop of wire between the poles of a magnet. Whenever a loop of wire is turned between the two poles of a magnet, the magnetism pushes the electrons that are already in the wire around and around the loop. As long as we keep the loop spinning, a current of electricity flows.

If only one loop of wire is spun between the poles of a magnet, the current is very feeble. If you loop the wire around twice, as shown in Figure 114, the magnet acts on twice as much of the wire at the same time; so the current is stronger. If a very long piece of wire is used and is looped around many times, and the whole coil is spun rapidly between the poles of a powerful magnet, myriads of the electrons in the wire rush around and around the loops--a powerful current of electricity flows through the wire.

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.

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-FASHIONED TELEPHONES ARE RUNG. The old-fashioned 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.

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-glass window, although it lets most of the light through.

306. There is a layer of coarse, 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.