Correction: John points out that Hall much earlier saw this, though I
assume he also saw the "anomalous" case of the charge carriers being
positive (I think aluminum is an example). I don't recall what material(s?)
Tolman and Stewart used, but I'm guessing it was copper, because they spun
and then stopped abruptly a coil of (presumably copper) wire, and measured
the sign of the potential difference from one end of the spool to the other.