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198 THE BOY ELECTRICIAN

is connected to the primary posts and the interrupter is
in operation a powerful shock will be felt. The shock may
be regulated from a weak current that can hardly be felt
to a very powerful one by providing the coil with a piece
of iron tubing of about seven-eighths of an inch inner
diameter and two inches long which Will slip on and ofi the
coil. When the tube is all the way on, the shock is very
mild, and when all the way off, the shock is very strong.
bf course any intermediate strength may be secured at
stages between the two extremes. '

The current from medical coils is often prescribed by
physicians for rheumatism and nervOus disorders, but must
be properly applied. The coil just described is harmless.
It will give a strong shock, but the only result is to make the
person receiving it drop the handles and not be anxious to
try it again.

Spark-Coils

A’ “ spark-co' ” is one of the most interesting pieces of
apparatus an experimenter can possess. The experiments
that may be performed with its aid are varied and many.

The purpose of a “ spark-coil ” is to generate enormously
high voltages which are able to send sparks across an air
space that .ordinary currents of low voltage could not pos-
sibly pierce. The spark-coil is the same in principle as the
small induction coils used as medical or shocking coils, but
is made on a larger scale and is provided with a condenser
connected across the terminals of the interrupter.

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