[rotorwrench]

I would use the wire terminal that comes from the ballast resistor after the ignition switch to check the amps just to insure a good result every time. Kevin is correct about either wire but I don't consider them both to be primary. One side is the breaker side after the primary and the other is power in to the primary after the ignition switch. This way folks don't get confused. The main thing is to measure the current draw so as long as it gets done, that's what matters.

Just to add: One terminal of a coil is attached to both the primary and the secondary where the other terminal is attached only to the primary. This has an affect on polarity of the coil and is what makes them different.