I was working in an auto shop years ago and a farmer wanted to upgrade his old tractor (Farmall A or something) to 12 negative ground with an alternator. The alternator had an internal regulator so all we had to do was hook up one wire on the alternator to hot, swap the leads on the ammeter so charge was charge instead of discharge, and get some 12 volt lights. I was arguing with the boss, claiming that the starter would spin backwards because switched polarity, he said do it anyway, and it worked. I guess I was thinking of the little motors with permanent magnets reversing direction with a polarity switch, but apparently the stator fields in a starter reversed their field too, so it worked. I guess.