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.

Fig. 111.

Fig. 111. A wet battery of three cells connected to ring a bell.

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.

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.

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 pass 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 glass; 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.

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 pass 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 glass or porcelain knobs. The reason is simple: Copper and practically all other metals are very good conductors of electricity; that is, they allow electricity to pass through them very easily. Cloth, rubber, glass, 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.

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 glass 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.

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?

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?

Inference Exercise

Explain the following:

311. A red postage stamp looks greenish gray in the green light of a mercury-vapor lamp.

312. Cracks are left between sections of the roadbed in cement auto highways.

313. Electricity goes up a mountain through a wire.

314. It is impossible to stand sidewise against a wall on one foot, when that foot touches the wall.

315. A charged storage battery will run an electric automobile.

316. An empty house is noisier to walk in and talk in than is a furnished one.

317. Lightning rods are made of metal.

318. It is harder to hold a frying pan by the end of the handle than by part of the handle close to the pan.

319. Diamonds flash many colors.

320. In swimming, if you have hold of a fastened rope and try to pull it toward you, you go toward it.

Section 35. Complete circuits.

Why does a doorbell ring when you push a button?

Why is it that when you touch one electric wire you feel no shock, while if you touch two wires you sometimes get a shock?

When a wire is broken in an electric light, why does it not light?

Suppose you have some water in an open circular trough like the one shown in Figure 123. Then suppose you have a paddle and keep pushing the water to your right from one point. The water you push pushes the water next to it, that pushes the water next to it, and so on all around the trough until the water just behind your paddle pushes in toward the paddle; the water goes around and around the trough in a complete circuit. There never is too much water in one place; you never run out of water. But then suppose a partition is put across the trough somewhere along the circuit. When the water reaches that, it cannot pass; it has no place to flow to, and the current of water stops.

The electric circuit. The flow of electricity in an electric circuit may be compared to the flow of the water in the tank we have been imagining. The long loop of wire extending out from the dynamo to your house and back again corresponds to the tank. The electricity corresponds to the water. Your dynamo pushes the electricity around and around the circuit, as the paddle pushes the water. But let some one break the circuit by putting a partition between two parts of it, and the electricity immediately stops flowing. One of the most effective partitions we can put into an electric circuit is a gap of air. It is very difficult for any electricity to flow through the air; so if we simply cut the wire in two, electricity can no longer flow from one part to the other, and the current is broken.

Breaking and making the circuit. The most convenient way to put an air partition into an electric circuit and so to break it, or to close the circuit again so it will be complete, is to use a switch.

Experiment 67. In the laboratory, examine the three different kinds of switches where the electricity flows into the lamp and resistance wire and then out again. Trace the path the electricity must take from the wire coming into the building down to the first switch that it meets; then from one end of the wire through the brass or copper to which the wire is screwed, through the switch and on out into the end of the next piece of wire. Turn the first switch off and see how a partition of air is made between the place where the electricity comes in and the place where it would get out if it could. Turn the switch on and notice how this gives the electricity a complete path through to the next piece of wire. In this way follow the circuit on through all the switches to the electric lamp.

If you examine the socket into which the lamp screws and examine the lamp itself, you will see that electricity which goes to the outer part of the socket passes into the rim of the lamp; from here it goes into one end of the filament. It passes through the filament to the other end, which is connected to the little brass disk at the end of the lamp. From this you can see that it goes into the center point of the socket, and then on into the second wire that connects to the socket. Trace the current on back through this other wire until you see where this wire leads toward the dynamo. You should understand that the electric lamp, the switches, the fuses, all things along the circuit, are simply parts of the long loop from the dynamo, as shown in Figure 124.

Connecting in parallel. The trouble with Figure 124 is that it is a little too simple. From looking at it you might think that the loop entered only one building. And it might seem that turning off one switch would shut off the electricity all along the line. It would, too, if the circuit were arranged exactly as shown above. To avoid this, and for other reasons, the main loop from the dynamo has branches so that the electricity can go through any or all of them at the same time and so that shutting off one branch will not affect the others. Electricians call this connecting in parallel; there are many parallel circuits from one power house.

Figure 125 illustrates the principle just explained. As there diagrammed, the electricity passes out from the dynamo along the lower wire and goes down the left-hand wire of circuit A through one of the electric lamps that is turned on, and then it goes back through the right-hand wire of the A circuit to the upper wire of the main circuit and then on back to the dynamo. But only a part of the electricity goes through the A circuit; part goes on to the B circuit, and there it passes partly through the electric iron. Then it goes back through the other wire to the dynamo. No electricity can get through the electric lamp on the B circuit, because the switch to the lamp is open. The switch on the C circuit is open; so no electricity can pass through it.

The purpose of the diagram is to show that electricity from the dynamo may go through several branch circuits and then get back to the dynamo, and that shutting off the electricity from one branch circuit does not shut it off from the others. And the purpose of this section is to make it clear that electricity can flow only through a complete circuit; it must have an unbroken path from the dynamo back to the dynamo again or from one pole of the battery back to the other pole. If the electricity does not have a complete circuit, it will not flow.

Application 52. A small boy disconnected the doorbell batteries from the wires that ran to them, and when he wanted to put the wires back, he could not remember how they had been connected. He tried fastening both wires to the carbon part of the battery, connecting one wire to the carbon and one to the zinc, and connecting both to the zinc. Then he decided that one wire was all that had to be connected anyway, that the second was simply to make it stronger. Which of the ways he tried, if any, would have been right?

Application 53. Dorothy was moving. "When they took out our telephone," she said to her chum, Helen, "the electrician just cut the wires right off."

"He must have turned off the electricity first," Helen answered, "or else it would all have run out of the cut ends of the wire and gone to waste."

"Why, it couldn't," Dorothy said. "Electricity won't flow off into the air."

"Of course it can if there is nothing to hold it in," Helen argued.

Which was right?

Inference Exercise

Explain the following:

321. It is very easy to get chilled when one is perspiring.

322. Ice cream becomes liquid if you leave it in your dish too long.

323. You should face forward when alighting from a street car.

324. There are always at least two electric wires going into a building that is wired.

325. Woolen sweaters keep you warm.

326. Steel rails are not riveted to railroad ties but the spikes are driven close to each rail so that the heads hook over the edge and hold the rail down without absolutely preventing its movement forward and backward. Why should rails be laid in this way?

327. The earth keeps whirling around the sun without falling into it, although the pull from the sun is very great.

328. Electricity is brought down from power houses in the mountains by means of cables.

329. White clothes are cooler than black when the person wearing them is out in the sun.

330. All the street cars along one line are stopped when a trolley wire breaks.

Section 36. Grounded circuits.

Why can a bird sit on a live wire without getting a shock, while a man would get a shock if he reached up and took hold of the same wire?

We have just been laying emphasis on the fact that for electricity to flow out of a dynamo or battery, it must have a complete circuit back to the battery or dynamo. Yet only one wire is needed in order to telegraph between two stations. Likewise, a single wire could be made to carry into a building the current for electric lights. This is because the ground can carry electricity.

If you make all connections from a battery or dynamo just as for any complete circuit, but use the earth for one wire, the electricity will flow perfectly well (Fig. 127). To connect an electric wire with the earth, the wire must go down deep into the ground and be well packed with earth; but since water pipes go down deep and the earth is already packed around them, the most convenient way to ground a circuit is to connect the wire that should go into the ground with the water pipe. The next experiment, the grounding of a circuit, should be done by the class with the help of the teacher.

Experiment 68. Caution: Keep the switches turned off throughout this experiment.⁶

Footnote 6: All through this chapter it is assumed that the electrical apparatus described in the appendix is being used. In this apparatus all the switches are on one wire, the other wire being alive even when the switches are turned off.

(a) Put a piece of fuse wire across the fuse gap. Screw the plug with nails in it into the lamp socket. Connect the bare end of a piece of insulated wire to the water faucet and touch the other end to one nail of the plug. If nothing happens, touch it to the other nail instead. The electricity has gone down into the ground through the water pipe, instead of into the other wire. The ground carries the electricity back to the dynamo just as a wire would.

(b) Put a new piece of fuse wire across the gap. Keep switches turned off. Touch the brass disk at the bottom of an electric lamp to the nail which worked, and touch the wire from the faucet to the other brass part of the lamp (Fig. 129). What happens?

Caution: Under no circumstances allow the switch to be turned on while you are doing any part of this experiment. Under no circumstances touch the wire from the faucet to the binding posts of the fuse gap. Do only as directed. Explain what would happen if you disobeyed these rules.

Why a bird is not electrocuted when it sits on a live wire. If a man accidentally touches a live wire that carries a strong current of electricity he is electrocuted; yet birds perch on such a wire in perfect safety. If a man should leap into the air and grasp a live wire, hanging from it without touching the ground, he would be no more hurt by it than a bird is. A person who is electrocuted by touching such a wire must at the same time be standing on the ground or on something connected with it. The ground completes the electric circuit which passes through the body. An electric circuit can always be completed through the ground, and when this is done, it is called grounding a circuit.

Application 54. Explain why only one wire is needed to telegraph between two stations; why you should not turn an electric light on or off while standing in a tub of water.

Application 55. In a house in the country, the electric wires passed through a double wall. They were separated from each other and well covered with insulation, but they were not within an iron pipe, as is now required in many cities. The current was alternating. One night when the lights were out a rat in the wall gnawed through the insulation of the wire and also gnawed clear through one of the wires. Did he get a shock? The next morning, the woman of the house wanted to use the electric iron in the kitchen and it would not work. The kitchen had in it a gas stove, a sink with running water, a table, a couple of chairs, and the usual kitchen utensils. There was also a piece of wire about long enough to reach across the kitchen. The electrician could not come out for several hours, and the woman wanted very much to do her ironing. Figure 130 is a diagram of the wires and the kitchen. Show what the woman might have done in order to use her iron until the electrician arrived.

Application 56. A man wanted to change the location of the wiring in his cement-floored garage. While he was working, would it have been best for him to stand on the bare cement floor, on a wire mat, on an old automobile tire, on a wet rug, or on some skid chains that were there?

Inference Exercise

Explain the following:

331. An ungreased wheel squeaks.

332. Lightning rods extend into the earth.

333. A banjo player moves his fingers toward the drum end of the banjo when he plays high notes.

334. When the filament breaks, an electric lamp will no longer glow.

335. An inverted image is formed by the lens of a camera.

336. A blown-out fuse may be replaced temporarily with a hairpin or with a copper cent.

337. Sparks fly when a horse's shoe hits a stone.

338. A room requires less artificial light if the wall paper is light than if it is dark.

339. Phonographs usually have horns, either inside or outside.

340. An electric car needs only one wire to make it go.

Section 37. Resistance.

What makes an electric heater hot?

Why does lightning kill people when it strikes them?

What makes an electric light glow?

We have talked about making electricity work when it flows in a steady stream, and everybody knows that it makes lights glow, makes toasters and electric stoves hot, and heats electric irons. But did it ever strike you as remarkable that the same electricity that flows harmlessly through the wires in your house without heating them, suddenly makes the wire in your toaster or the filament in your incandescent lamp glowing hot? The insulation is not what keeps the wire cool, as you can see by the next experiment.

Experiment 69. Between two of the laboratory switches you will find one piece of wire which has no insulation. Turn on the electricity and make the lamp glow; see that you are standing on dry wood and are not touching any pipes or anything connected to the ground. Feel the bare piece of wire with your fingers. Why does this not give you a shock? What would happen if you touched your other hand to the gas pipe or water pipe? Do not try it! But what would happen if you did?

The reason that the filament of the electric lamp gets white hot while the copper wire stays cool is this: All substances that conduct electricity resist the flow somewhat; there is something like friction between the wire and the electricity passing through it. The smaller around a wire is, the greater resistance it offers to the passing of an electric current. The filament of an electric lamp is very fine and therefore offers considerable resistance. However, if the filament were made of copper, even as fine as it is, it would take a much greater flow of electricity to make it white hot, and it would be very expensive to use. So filaments are not made of copper but of substances which do not conduct electricity nearly as well and which therefore have much higher resistance. Carbon was once used, but now a metal called tungsten is used for most incandescent lamps. Both carbon and tungsten resist an electric current so much that they are easily heated white hot by it. On the other hand, they let so little current through that what does pass flows through the larger copper wires very easily and does not heat them noticeably.

Experiment 70. Turn on the switch that lets the electricity flow through the long resistance wire that passes around the porcelain posts. Watch the wire.

The resistance wire you are using is an alloy, a mixture of metals that will resist electricity much more than ordinary metals will. This is the same kind of wire that is used in electric irons and toasters and heaters. It has so great a resistance to the electricity that it is heated red hot, or almost white hot, by the electricity passing through it.

Application 57. A power company wanted to send large quantities of electricity down from a mountain. Should the company have obtained resistance wire or copper wire to carry it? Should the wire have been large or fine?

Application 58. A firm was making electric toasters. In the experimental laboratory they tried various weights of resistance wire for the toasters. They tried a very fine wire, No. 30; a medium weight wire, No. 24; and a heavy wire, No. 18. One of these wires did not get hot enough, and it took so much electricity that it would have been too expensive to run; another got so hot that it soon burned out. One worked satisfactorily. Which of the three sizes burned out? Which was satisfactory?

Inference Exercise

Explain the following:

341. If you attach one end of a wire to a water faucet and connect the other end to an electric lamp in place of one of the regular lighting wires, the lamp will light.

342. The needle of a sewing machine goes up and down many times to each stroke of the treadle.

343. Trolley wires are bare.

344. If you had rubbers on your feet, you could take hold of one live wire with perfect safety, provided you touched nothing else.

345. If you were on the moon, you would look up at the earth.

346. Toy balloons burst when they go high up where the air is thin.

347. You have to put on the brakes to stop a car quickly.

348. Telephone wires are strung on glass supporters.

349. If you pour boiling water into a drinking glass, frequently the glass will crack.

350. An asbestos mat tends to keep food from burning.

Section 38. The electric arc.

How can electricity set a house on fire?

This is one of the most important sections in the book.

Do you know that you can make an arc light with two ordinary pencils? The next experiment, which should be done by the class with the help of the teacher, shows how to do it.

Experiment 71. Sharpen two pencils. About halfway between the point and the other end of each pencil cut a notch all the way around and down to the "lead," or burn a notch down by means of the glowing resistance wire. What you call the "lead" of the pencil is really graphite, a form of carbon. The leads of your two pencils are almost exactly like the carbons used in arc lights, except, of course, that they are much smaller. Turn off the electricity both at the snap switch and at the knife switch. Fasten the bare end of a 2-foot piece of fine insulated wire (about No. 24) around the center of the lead in each pencil so that you get a good contact, as shown in Figure 132. Fasten the other bare end of each wire to either side of the open knife switch so that when this switch is open the electricity will have to pass down one wire to the lead of one pencil, from that to the lead of the other pencil, and from that back through the second wire to the other side of the knife switch and on around the circuit, as shown in Figure 133. Keep the two pencils apart and off the desk, while some one turns on the snap switch and the "flush" switch that lets the electricity through the resistance wire. Now bring the pencil points together for an instant, immediately drawing them apart about half an inch. You should get a brilliant white arc light.

Caution: Do not look at this brilliant arc for more than a fraction of a second unless you look through a piece of smoked or colored glass.

Blow out the flame when the wood catches fire. After you have done this two or three times, the inside of the wood below the notches will be burned out so completely that you can pull it off with your fingers, leaving the lead bare all the way up to the wires.

Let the class stand well back and watch the teacher do the next part of the experiment.

Connect two heavy insulated copper wires, about No. 12, to the sides of the knife switch just as you connected the fine wires. But this time bring the ends of the copper wires themselves together for an instant, then draw them apart. Hold the ends of the wires over the zinc of the table while you do this, as melted copper will drop from them.

What happens when an arc is formed. What happens when you form an electric arc is this: As you draw the two ends of the pencils apart, only a speck of the lead in each touches the other. The electricity passing for an instant through the last speck at the end of the pencil makes it so hot that it turns to vapor. The vapor will let electricity go through it, and makes a bridge from one pencil point to the other. But the vapor gets very hot, because it has a rather high resistance. This heat vaporizes more carbon and makes more vapor for the electricity to pass through, and so on. The electricity passing through the carbon vapor makes it white hot, and that is what causes the brilliant glow. Regular arc lights are made exactly like this experimental one, except that the carbons used are much bigger and are made to stand the heat better than the small carbons in your pencil.

Carbon is one of those substances that turn directly from a solid to a gas without first melting. That is one reason why it is used for arc lights. But copper melts when it becomes very hot, as you saw when you made an arc light with the copper wires. So copper cannot be used for practical arc lights.

Fires caused by arcs. There is one extremely important point about this experiment with arcs: most fires that result from defective wiring are caused by the forming of arcs. You see, if two wires touch each other while the current is passing and then move apart a little, an arc is formed. And you have seen how intensely hot such an arc is. Two wires rubbing against each other, or a wire not screwed tightly to its connection, can arc. A wire broken, but with its ends close enough together to touch and then go apart, can cause an arc. And an arc is very dangerous in a house if there is anything burnable near it.

Wires should never be just twisted together and then bound with tape to form a joint. Twisted wires sometimes break and sometimes come loose; then an arc forms, and the house catches fire. Good wiring always means soldering every joint and screwing the ends of the wires tightly into the switches or sockets to which they lead.

Keeping arcs from forming. Well-wired houses have the wires brought in through iron pipes, called conduits, and the conduits are always grounded; so if an arc should form anywhere along the line, the house would be protected by an iron conduit and if one of the loose ends of wire came in contact with the conduit, the current would rush to the ground through it, blowing out a fuse. The next section tells about the purpose of fuses.

The directions that usually come with electric irons, toasters, and stoves say that the connection should be broken by pulling out the plug rather than by turning off the switch. This is because the switch in the electric-light socket sometimes loses its spring and instead of snapping all the way around and quickly leaving a big gap, it moves only a little way around and an arc is formed in the socket; if you hear a sizzling sound in a socket, you may be pretty sure that an arc has been formed. But when you pull the plug entirely out of the iron or stove, the gap is too big for an arc to form and you are perfectly safe.

Fire commissions usually condemn extension lights, because if the insulation wears out on a lamp cord so that the two wires can come in contact, a dangerous arc may easily form. And the insulation might suddenly be scraped off by something heavy moving across the cord. This can happen whether the light at the end of the cord is turned on or off. So it is best if you have an extension light always to turn it off at the socket from which the cord leads, not at the lamp itself. Many people do not do this, and go for years without having a fire. But so might you live for years with a stick of dynamite in your bureau drawer and never have an explosion. Still, it is not wise to keep dynamite in your bureau.

Arc lights themselves, of course, are no more dangerous than is a fire in a kitchen stove. For an arc light is placed in such a way that nothing can well come near it to catch fire. The danger from the electric arc is like the danger from gasoline spilled and matches dropped where you are not expecting them, so that you are not protected against them.

Fortunately ordinary batteries have not enough voltage to cause dangerous arcs. So you do not have to be as careful in wiring for electric bells and telegraph instruments. It requires the high voltage of a city power line to make a dangerous electric arc.

So many fires are caused by electric arcs forming in buildings, that you had better go back to the beginning of this section and read it all through again carefully. It may save your home and even your life.

After you have reread this section, test your understanding of it by answering the following questions:

1. How can you make an electric arc?

2. Why should wires not be twisted together to make electric connections?

3. Why should wires be brought into houses and through walls in iron conduits?

4. Why should you pull out the plug of an electric iron, percolator, toaster, heater, or stove?

5. Why do fire commissions condemn extension lights?

6. If you use an extension light, where should it be turned off?

7. If you hear a sizzling and sputtering in your electric-light socket, what does it mean? What should you do?

8. Is there any danger in defective sockets with switches that do not snap off completely? What is the danger?

9. In Application 55, page 228, if the rat had gnawed the wire in two while the electric iron was being used, would anything have happened to the rat? Would there have been any danger to the house?

10. Where a wire is screwed into an electric-light socket, what harm, if any, might result from not screwing it in tightly?

11. How can a wire be safely spliced?

12. Why is an electric arc in a circuit dangerous?

Inference Exercise

Explain the following:

351. White objects look blue when seen through a blue glass.

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 passes 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.

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 passes 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 pass 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.

Unfortunately, however, the fuse is almost no protection against an electric arc. The copper vapor through which the electricity passes 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 pass 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 pass 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 passing 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 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:

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.

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.

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.

Here is the Morse telegraph code in dots and dashes:

Letters

Numerals

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 passes 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 passing 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 passing 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 passes 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.

The electric bell, like the telephone and telegraph, works on the simple principle that electricity flowing through a wire that is wrapped around and around a piece of iron will turn that piece of iron into a magnet as long as the electricity flows.

The electric motor. The motor of a street car is a still more complicated carrying out of the same principle. In the next experiment you will see the working of a motor.

Experiment 79. Connect the wires from the laboratory battery to the two binding posts of the toy motor, and make the motor run. Examine the motor and see that it is made of several electromagnets which keep attracting each other around and around.

Motors, and therefore all things that are moved by electricity, including trolley cars and electric railways, submarines while submerged, electric automobiles, electric sewing machines, electric vacuum cleaners, and electric player-pianos, are moved by magnetizing a piece of iron and letting this pull on another piece of iron. And the iron is magnetized by letting a current of electricity flow around and around it.

Inference Exercise

Explain the following:

371. When a fuse blows out, you can get no light.

372. If you lay your ear on a desk, you hear the sounds in the room clearly.

373. If you touch a live wire with wet hands, you get a much worse shock than if you touch it with dry hands.

374. A park music stand is backed by a sounding board.

375. The clapper of an electric bell is pulled against the bell when you push the button.

376. A hot iron tire put on a wagon wheel fits very tightly when it cools.

377. Candy will cool more rapidly in a tin plate than in a china plate.

378. When a trolley wire breaks and falls to the ground it melts and burns at the point at which it touches the ground.

379. By allowing the electricity from the trolley wire to flow down through an underground coil of wire, a motorman can open a switch in the track.

380. The bare ends of the two wires leading to your electric lamp should never be allowed to touch each other.