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.

FIG. 4. - SOLENOID.