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.