f any metal, and turn on the electricity.
What happens? Examine the pieces of the fuse wire that are
left.

It was so easy for the electricity to pass through the nails and wire,
that it gushed through at a tremendous rate. This melted the fuse
wire, or blew out the fuse. If the fuse across the gap by the socket
had not been the more easily burned out, one or perhaps both of the
more expensive fuses up above, where the wire comes in, would have
blown out. These cost about 10 cents each to replace, while the fuse
wire you burned out costs only a fraction of a cent. If there were
no fuses in the laboratory wirings and you had "short circuited" the
electricity (given it an easy enough path), it would have blown
out the much more expensive fuses where the electricity enters the
building. If there were no big fuses where the electricity enters
the building, the rush of electricity would make all the copper wires
through which it flowed inside the building so hot that they would
melt and set fire to the building. As long as you keep a piece of fuse
wire across the gap, there is no danger from short circuits.

WHY FUSE WIRE MELTS. For two reasons, the fuse wire melts when
ordinary wire would not. First, it has enough resistance to
electricity so that if many amperes (much current) flow through, it
gets heated. It has not nearly as much resistance, however, as the
filament in an electric lamp or even as has the long resistance wire.
It does not become white hot as they do.

Second, it has a low melting point. It melts immediately if you hold a
match to it; try this and see. Consequently, long before the fuse wire
becomes red hot, it melts in two. It has enough resistance to make it
hot as soon as too many amperes flow through; and it has such a low
melting point that as soon as it gets hot it melts in two, or blows
out. This breaks the circuit, of course, so that no more electricity
can flow. In this way the fuse protects houses from catching fire
through short circuits.

[Illustration: FIG. 137. What will happen when the pin is thrust
through the cords and the electricity turned on?]

Unfortunately, however, the fuse is almost no protection against an
electric arc. The copper vapor through which the electricity passes
in an arc has enough resistance to keep the amperage (current) low;
so the arc may not blow out the fuse at all. But if it were not for
fuses, there would be about as much danger of houses being set