er, or a wire not screwed tightly to
its connection, can arc. A wire broken, but with its ends close enough
together to touch and then go apart, can cause an arc. And an arc is
very dangerous in a house if there is anything burnable near it.

Wires should never be just twisted together and then bound with tape
to form a joint. Twisted wires sometimes break and sometimes come
loose; then an arc forms, and the house catches fire. Good wiring
always means soldering every joint and screwing the ends of the wires
tightly into the switches or sockets to which they lead.

[Illustration: FIG. 135. An arc lamp. The carbons are much larger than
the carbons in the pencils, and the arc gives an intense light.]

KEEPING ARCS FROM FORMING. Well-wired houses have the wires brought
in through iron pipes, called _conduits_, and the conduits are always
grounded; so if an arc should form anywhere along the line, the house
would be protected by an iron conduit and if one of the loose ends of
wire came in contact with the conduit, the current would rush to the
ground through it, blowing out a fuse. The next section tells about
the purpose of fuses.

The directions that usually come with electric irons, toasters, and
stoves say that the connection should be broken by pulling out the
plug rather than by turning off the switch. This is because the switch
in the electric-light socket sometimes loses its spring and instead
of snapping all the way around and quickly leaving a big gap, it moves
only a little way around and an arc is formed in the socket; if you
hear a sizzling sound in a socket, you may be pretty sure that an arc
has been formed. But when you pull the plug entirely out of the iron
or stove, the gap is too big for an arc to form and you are perfectly
safe.

Fire commissions usually condemn extension lights, because if the
insulation wears out on a lamp cord so that the two wires can come
in contact, a dangerous arc may easily form. And the insulation might
suddenly be scraped off by something heavy moving across the cord.
This can happen whether the light at the end of the cord is turned on
or off. So it is best if you have an extension light always to turn it
off at the socket from which the cord leads, not at the lamp itself.
Many people do not do this, and go for years without having a fire.
But so might you live for years with a stick of dynamite in your
bureau drawer and never have an explosion. Still, it is not wise to