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 cla.s.s with the help of the teacher.
[Ill.u.s.tration: 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 a.s.sumed 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 bra.s.s disk at the bottom of an electric lamp to the nail which worked, and touch the wire from the faucet to the other bra.s.s part of the lamp (Fig.
129). What happens?
_Caution: Under no circ.u.mstances allow the switch to be turned on while you are doing any part of this experiment. Under no circ.u.mstances 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.
[Ill.u.s.tration: 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 pa.s.ses through the body. An electric circuit can always be completed through the ground, and when this is done, it is called _grounding a circuit_.
[Ill.u.s.tration: 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 pa.s.sed 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.
[Ill.u.s.tration: 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?
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 pa.s.sing through it. The smaller around a wire is, the greater resistance it offers to the pa.s.sing 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 pa.s.s flows through the larger copper wires very easily and does not heat them noticeably.
[Ill.u.s.tration: 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 pa.s.ses 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 pa.s.sing through it.
_APPLICATION 57._ A power company wanted to send large quant.i.ties 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.