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20 THE BOY ELECTRICIAN

fill the partial vacuum, thus producing the terrifying sounds
called thunder.

In _the eighteenth century, electricity was believed to be
a sort of fiery atmospheric discharge, as has been said.
Later it was discovered that it seemed to flow like water
through certain mediums, and so was thought to be a fluid.
Modern scientists believe it to be simply a vibratory mo-
tion, either between adjacent particles or in the ether sur-
rounding" those particles.

It was' early discovered that electricity would travel
through some mediums but not through others. These
were. termed respectively “ conductors ” and “ non-con-
ductors ” or insulators. Metals such as silver, copper, gold,
and other substances like charcoal, water, etc., are good
conductors. Glass, silk, wool, oils, wax, etc. , are non-
conductors or insulators, while many other substances,
like wood, marble, paper, cotton, etc., are partial con-
ductors. '

There seems to be two kinds of electricity, one called
“ static ” and the other “ current ” electricity. The former
is usually produced by friction while the latter is generated
by batteries or dynamos. I

A very simple and well-known method of generating
static. electricity is by shufliing or sliding the feet over
the carpet. The body will then become charged, and if the
knuckles are presented to some metallic object, such as a
gas-jet or radiator, a stinging little spark will jump out to
meet it.

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