As the armature rotates in a magnetic field, it induces electricity.

Next, place the drain pan right below the plug, and then, use the 
wrench to carefully turn the plug in a counterclockwise motion up until the point 
that it can be rotated freely. The slip ring and external rheostat makes 
it possible to add external resistance to the rotor circuit, 
enabling them to have a higher resistance during starting and thus higher starting torque.