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.

[Illustration: 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 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 touc