100
The Body Electric
anesthesia findings, and with the whole-body monitoring setup I also
was able to correlate the entire pattern of surface voltages with the ani-
mal's level of activity while not anesthetized. Negative potentials in the
brain's frontal area and at the periphery of the nervous system were asso-
ciated with wakefulness, sensory stimuli, and muscle movements. The
more activity, the greater the negative potentials were. A shift toward
the positive occurred during rest and even more so during sleep. *
In my reading on solid-state electronics I found another way I could
test for current in the salamander. Luckily it was cheap and easy; I could
do it without buying more equipment. Best of all, it should work only
if the current was semiconducting.
Suppose you think you have a current flowing through some conduc-
tor—a salamander's limb, for instance. You put it in a strong magnetic
field so that the lines of force cut across the conductor at right angles.
Then you place another conductor, containing no current, perpendicular
to both the original conductor (the limb) and the magnetic field. If there
is a current in whatever you're testing, some of the charge carriers will
be deflected by the magnetic field into the other conductor, producing a
voltage that you can measure. This is called the Hall voltage, after the
gentleman who discovered it. The beauty of it is that it works dif-
ferently for the three kinds of current. For any given strength of mag-
*l didn't know it until later, but another experimenter named H. Caspers made
similar findings at about the same time.