By connecting one of the coils, A, with the battery by means of the
wires, the action of a helix or solenoid is shown. When so connected,
the helix will draw up with itself a barrel pen, or any light iron or
steel object. (See Fig. 4.) This is not a true solenoid, but it is
generally known by that name. In a true solenoid one of the terminals
is passed back through the center of the coil.
[Illustration: FIG. 4.--SOLENOID.]