124
The Body Electric
Despite the initimidating names, this device is fairly simple. It's a
filter that screens out either the positive (P) or the negative (N) part of a
signal. As mentioned in Chapter 4, current can flow through a crystal
lattice either as free electrons or as "holes" that can shift their positions
much as the holes migrate when you move the marbles in a game of
Chinese checkers. Since current can flow from a P-type to an N-type
semiconductor but not the other way, a junction of the two will filter, or
rectify, a current.
The phonograph would be impossible without this device. As the
diamond or sapphire crystal of the stylus rides a record's groove, the
groove's changing shape deforms it ever so slightly. The crystal, of
course, transforms the stresses into a varying electrical signal, which is
amplified until we can hear it. It would be an unintelligible hum, how-
ever, if we heard both the deformation pulse and the release pulse.
Therefore we place a rectifier in the circuit. It passes current in one
direction only, so the impulses don't cancel each other out. The signal is
rectified, and when we feed it to a loudspeaker we hear music. Bassett
and I felt sure we were seeing evidence of rectification in the fact that
bone's release pulse was much smaller than the one from stress.
RECTIFIED SIGNALS MAKE THE PHONOGRAPH POSSIBLE
If the negative piezoelectric signal stimulated growth, maybe we
could induce bone growth ourselves with negative current.* We tried
out the idea on eighteen dogs. In the thighbone of one hind leg we
implanted a battery pack. The electrodes were made of platinum, a non-
reactive metal, to minimize any possible electrochemical irritation, and
we inserted them through drill holes directly into the marrow cavity. As
controls, some of the devices didn't have a battery. After three weeks we
found that the active units had produced large amounts of new bone
*Again Dr.
Yasuda and his colleagues had already done so, but their results seem to
have been due to bone's ability to grow in response to irritation from the electrodes.
They used alternating current, which is now known to have no direct growth-stimulat-
ing effect.