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STATIC ELECTRIC MACHINES 47

so formed will just touch the sectors on the disks when the
latter are revolved.

These are ,the neutralizers and are arranged in the ap-
proximate positions shown in Figure 44.

The driving-wheels are connected to the bosses by means
of small round leather belts. The belt at the rear of the
machine is crossed in order to make the plates revolve in
opposite directions.

If the machine has been properly built it is now ready for
operation. It may be necessary to charge the machine the
first time that it is used by touching several of the sectors
with the charged cover of an electrophorus. Then if the
handle is turned the accumulated electricity should dis-
charge across the spark-gap at the top of the machine in
the form of bright blue sparks.

Experiments with an Electric Machine

Many interesting experiments can be performed with an
electric machine. The number is ahnost unlimited. A few
of the most instructive ones are described below. Others
can be found in almost any text book on physics.

The Leyden jar consists of a glass jar coated with tinfoil
part way up on both the outside and inside. Through the
wooden stopper passes a brass rod or a heavy copper wire
which connects with the inner coating of tinfoil by means
of a small brass chain. The upper and outside end of the
rod usually terminates in a brass ball or knob.‘

It is a very simple matter to make a good Leyden jar.

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