The object of the induction-coil is to produce high intensity, or
pressure, from a comparatively weak pressure and large current of
electricity; so, if we add still more wire, the magnet has a larger
number of turns to act upon and thus makes a very strong pressure, or
large number of volts, but a lesser number of ampères.
Instead of taking one piece of iron, as we would for an ordinary
electromagnet, we take a bundle of iron wires in making an
induction-coil, as these give a stronger effect. Around this bundle of
wires we wrap many turns of insulated copper wire. This is called the
_primary coil_, and the ends of this wire are to be attached to the
battery.
[Illustration: Fig. 9]
On top of, or over, this primary coil we wrap a great many turns of
very fine wire, of which, as it is so fine, a great length can be used.
This is called the _secondary coil_, and it is in this coil that the
volts, or pressure, of electricity become strongest.