At the left-hand side of the cut is a "circuit-breaker," which is
simply a piece of iron (armature) on a spring placed opposite the iron
core. This armature is made a part of the wire leading to the primary
coil. When the current from the battery is sent through the wires,
the core becomes magnetized and draws this armature away from a fixed
contact point, thus breaking the circuit, but the spring pulls it back,
again completing the circuit, and so it keeps going back and forth very
rapidly with a br-r-r-ing sound.
If you were now to take hold of the ends of the secondary coil you
would get a continuous series of quick shocks which would feel like
pins and needles running into you.
Perhaps most of you have taken hold of the handles of a medical battery
and have had shocks therefrom. In so doing, you have simply had the
current from the secondary of an induction-coil. The current may be
made weaker by sliding a metallic cover over part of the iron core and
so shutting off part of the magnetic effect.