CHAPTER EIGHT

Section 33.Making electricity flow.

What causes a battery to produce electricity?What makes electricity come into our houses?

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 numberof electrons; the other part has unusually few. The parts with unusually large numbers of electrons gather around the zinc; so the zinc isnegatively 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 ispositively 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.

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 runthrough 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 astorage battery, and it is much used in submarines and automobiles.

Fig. 112.Fig. 112.A battery of three dry cells.

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.

Fig. 113.Fig. 113.A storage battery.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 andthe 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 page199. 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.

Fig. 113.Fig. 113.A storage battery.

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 andthe 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 page199. 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 understanda 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.

Fig.Fig.114. Spinning loops of wire between the poles of a magnet causes a current of electricity to flow through the wire.

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.

Fig.Fig.115. The more loops there are, the stronger the current.

Now suppose you bring one loop of the long wire out, as shown in Figure 115, and suppose you spin the restof 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 thespinning coil of wire, as shown in Figure 116. Such an arrangement as this is really an extremely simple dynamo.

Fig.Fig.116. If the electricity passes through a lamp on its way around the circuit the filament of the lamp glows.

Fig.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-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.

Fig.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 calleddirect-current(d. c.) electricity. But it is easier to raise and lower the voltage (pressure) of the current if instead of being direct it isalternating; that is, if for one instant the electricityflows 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 calledalternating 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.

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-coloredgravel 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.

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-coloredgravel 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?

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 agood conductor. Anything that carries it poorly is called apoor conductor. Anything that allows practically no electricity to pass through it is called aninsulator.

Experiment 65.5Turn 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, page226, before doing this experiment.

Fig.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 besafe, 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. Thereason 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.

Fig.Fig.120. Will electricity go through the glass?

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 aninch 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.

Fig. 121.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.

Fig.Fig.122. Which should he choose to connect the broken wires?

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 youwork; 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?

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 youwork; 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?

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.

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.

Section35.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?

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 houseand 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.

Fig.Fig.123. Electricity flows around a completed circuit somewhat as water might be made to flow around this trough.

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; thenfrom 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 gothrough any or all of them at the same time and so that shutting off one branch will not affect the others. Electricians call thisconnecting in parallel; there are many parallel circuits from one power house.

Fig.Fig.124. Diagram of the complete circuit through the laboratory switches.

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 circuitAthrough one of the electric lamps that is turned on, and then it goes back through the right-hand wire of theAcircuit to the upper wire of the main circuit and then on back to the dynamo. But only a part of the electricity goes through theAcircuit; part goes on to theBcircuit, 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 theBcircuit, because the switch to the lamp is open. The switch on theCcircuit 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 sectionis 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?

Fig.Fig.125. Parallel circuits.

Fig.Fig.126. How should he connect them?

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?

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?

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.

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.

Section36.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 wellpacked 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.

Fig.Fig.127. The ground can be used in place of a wire to complete the circuit.

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.

(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.

Fig.Fig.128. Grounding the circuit. The faucet and water pipe lead the electricity to the ground.

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 calledgrounding a circuit.

Fig.Fig.129. How the lamp and wire are held to ground the 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 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.

Fig.Fig.130. How can the electric iron be used after one wire has been cut?

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?

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.

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.

Section37.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 ofcopper 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 calledtungstenis 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.

Fig.Fig.131. Feeling one live wire does not give her a shock, but what would happen if she touched the gas pipe with her other hand?

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 ofwire 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?

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?

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.

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.

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