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down flat, and the faint crackling noise, showing that
sparking is taking place as the comb passes through the
hair, will be plainly heard. The electricity is produced- by
the friction between the hair and the comb.

Electricity may be produced by friction between a num-
ber of substances. A hard rubber rod, a glass rod, a rubber
comb or a stick of sealing-wax may be very easily electri-
fied by rubbing them briskly with a piece of dry, warm

Electroscopes are devices for detecting the presence of
static electricity.

A very simple form of electroscope may be made in much
the same manner as the paper compass described in the




FIG. 31. — A Paper Electroscope.

last chapter. It may be cut out of writing-paper and
mounted on a pin stuck through a cork. If an electrified
rod is held near the electroscope it may be made to whirl
around in the same manner as a compass needle when a bar
magnet is brought to it.

The Pith-Ball Electroscope is a very simple device, in
Which a ball of cork or elder pith is hung by a fine silk
thread from an insulated support. A suitable electroscope

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