[rotorwrench]

The coils are made to be an inductor same as used in AC circuits. They use the breaker points to make them operate as an inductor under pulsating half wave DC or simulated AC. They do build a magnetic field but they don't hold it. An AC coil is used to demagnitise parts.

The two coils inside (primary and secondary) are connected together at one end and not the other. It's this connection that makes them polarity sensitive.