having a brass knob K connecting with its internal coat. When the
charged plate or conductor of the electrophorus touches the knob
the inner foil takes a positive charge, which induces a negative
charge in the outer foil through the glass. The corresponding
positive charge induced at the same time escapes through the hand
to the ground or "earth." The inner coating is now positively and
the outer coating negatively electrified, and these two opposite
charges bind or hold each other by mutual attraction. The bottle
will therefore continue charged for a long time; in short, until
it is purposely discharged or the two electricities combine by
leakage over the surface of the glass.

To discharge the jar we need only connect the two foils by a
conductor, and thus allow the separated charges to combine. This
should be done by joining the OUTER to the INNER coat with a stout
wire, or, better still, the discharging tongs T, as shown in the
figure. Otherwise, if the tongs are first applied to the inner
coat, the operator will receive the charge through his arms and
chest in the manner of Cuneus and Muschenbroeck.

Leyden jars can be connected together in "batteries," so as to
give very powerful effects. One method is to join the inner coat
of one to the outer coat of the next. This is known as connecting
in "series," and gives a very long spark. Another method is to
join the inner coat of one to the inner coat of the next, and
similarly all the outer coats together. This is called connecting
"in parallel," or quantity, and gives a big, but not a long spark.

Of late years the principle of induction, which is the secret of
the Leyden jar and electrophorus, has been applied in constructing