And here's a related thought: What's the drift velocity in the bearing race? Wiki's article on drift velocity gives one actual number, which is 0.00028 meters per second for a copper wire 1 mm cross section carrying 3 amps. I'm not going to try to compute anything right now so I'll just take that as "the drift velocity": 0.028 cm/sec.
Resistance is the result of "drifting" electrons running into stuff, as I understand it. Race 2.5 cm in radius rotating at 600 rpm is going 10*2.5*pi = 60 cm/sec, or some huge factor larger than the drift velocity. OK, now here's the real question: What's the resistance of a wire moving longitudinally? Is it higher for currents traveling with the motion than for currents traveling against the motion? (I.e., is it lower for currents in which the electrons are being "carried along" by the motion and don't need to drift much, or maybe don't need to drift at all?) If so then that could be playing a role here, for the case where the bearing has its negative contact on the outer race and positive on the inner.
