.

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.

[Illustration: FIG. 136. _A_, the "fuse gap" and _B_, the "nail
plug."]

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 o