Life's Potentials
101
netic field, the Hall voltage is proportional to the mobility of the charge
carriers. Ions in a solution are relatively big and barely deflected by the
field. Electrons in a wire are constrained by the nature of the metal. In
both cases the Hall voltage is small and hard to detect. Electrons in
semiconductors are very free to move, however, and produce Hall volt-
ages with much weaker fields.
After finding a C-shaped permanent magnet, an item not much in
demand since the advent of electromagnets, I set up the equipment. I
took a deep breath as I placed the first anesthetized salamander on its
plastic support, with one foreleg extended. I'd placed electrodes so that
they touched the limb lightly, one on each side, and I'd mounted the
magnet so as to swing in with its poles above and below the limb, close
to yet not touching it. I took voltage measurements every few minutes,
with the magnet and without it, as the animal regained consciousness. I
also measured the DC voltage from the tips of the fingers to the spinal
cord. In deep anesthesia, the DC voltage along the limb was zero and so
was the Hall voltage. As the anesthetic wore off, the normal potential
along the limb gradually appeared, and so did a beautiful Hall voltage.
It increased right along with the limb potential, until the animal re-
covered completely and walked away from the apparatus. The test
worked every time, but I don't think I'll ever forget the thrill of watch-
ing the pen on the recorder trace out the first of those Hall voltages.
THE HALL EFFECT