STARTED in the wire B (fig. 32) in direction of the arrow, it will
induce or give rise to a momentary current in the wire A, flowing
in a contrary direction to itself. Again, if the current in B be
STOPEED, a momentary current is set up in the wire A in a
direction the same as that of the exciting current in B. While the
current in B is quietly flowing there is no induced current in A;
and it is only at the start or the stoppage of the inducing or
PRIMARY current that the induced or SECONDARY current is set up.
Here again we have the influence of the magnetic field around the
wire conveying a current.

This is the principle of the "induction coil" so much employed in
medical electricity, and of the "transformer" or "converter" used
in electric illumination. It consists essentially, as shown in
figure 35, of two coils of wire, one enclosing the other, and both
parallel or concentric. The inner or primary coil P C is of short
thick wire of low resistance, and is traversed by the inducing
current of a battery B. To increase its inductive effect a core of
soft iron I C occupies its middle. The outer or secondary coil S C
is of long thin wire terminating in two discharging points D1 D2.
An interrupter or hammer "key" interrupts or "makes and breaks"
the circuit of the primary coil very rapidly, so as to excite a
great many induced currents in the secondary coil per second, and
produce energetic sparks between the terminals D1 D2. The
interrupter is actuated automatically by the magnetism of the iron
core I C, for the hammer H has a soft iron head which is attracted
by the core when the latter is magnetised, and being thus drawn
away from the contact screw C S the circuit of the primary is
broken, and the current is stopped. The iron core then ceases to
be a magnet, the hammer H springs back to the contact screw, and