[rotorwrench]

The old coil testers from back in the day would start out with a warming cycle to test the coil as it would be in operation. A simple Ohms check of the primary and secondary will generally be OK when the internal coils are cold but after they heat up and expand, they will lose continuity if one strand of either coil is cracked.

An inductor or more specifically an ignition coil operating on DC current will not function well at all without the aid of a capacitor in the breaker system. The breaker makes the current into pulsating DC so that the magnetic flux in the coil core can build and collapse. The condenser rapidly stores up and utilizes the back flow generated by the collapse and induction process other wise the points would rapidly burn up. The loss of that back flow makes for a spark that is almost non-existent. Polarity is also very important to the coils function.

A person has to take care to remember to open the electrical circuit to the coil by shutting off the ignition switch any time it's turned on. If a person forgets and the points happen to be closed then the coil will heat up like any electromagnet will and this can burn the primary coil up in relatively short order.