the current again flows in the primary circuit only to be
interrupted again as before. In this way the current in the
primary coil is rapidly started and stopped many times a second,
and this, as we know, induces corresponding currents in the
secondary which appear as sparks at the discharging points. The
effect of the apparatus is enhanced by interpolating a "condenser"
C C in the primary circuit. A condenser is a form of Leyden jar,
suitable for current electricity, and consists of layers of
tinfoil separated from each other by sheets of paraffin paper,
mica, or some other convenient insulator, and alternate foils are
connected together. The wires joining each set of plates are the
poles of the condenser, and when these are connected in the
circuit of a current the condenser is charged. It can be
discharged by joining its two poles with a wire, and letting the
two opposite electricities on its plates rush together. Now, the
sudden discharge of the condenser C C through the primary coil P C
enhances the inductive effect of the current. The battery B, here
shown by the conventional symbol [Electrical Symbol] where the
thick dash is the negative and the thin dash the positive pole, is
connected between the terminals T1 T2, and a COMMUTATOR or pole-
changer R, turned with a handle, permits the direction of the
current to be reversed at will.

Figure 36 represents the exterior of an ordinary induction coil of
the Ruhmkorff pattern, with its two coils, one over the other C,
its commutator R, and its sparkling points D1D2, the whole being
mounted on a mahogany base, which holds the condenser.

The intermittent, or rather alternating, currents from the
secondary coil are often applied to the body in certain nervous