'A' or 'B' circuit generator?

This is so embarrassing, I must just be getting old; I can't remember
if the generator in my '61 C-172 is an A or B circuit. Worse, I
forget which one is which. Does the 'A' supply controlled voltage
to the field and the 'B' does a controlled "ground out" of
voltage coming from the field or do I have it backwards? My system
seems to be charging, just not enough. It might be OK and I'm just
taking too many short hops with too many bells and whistles on. Can
someone refresh my memory so my checks are correct?

Jim