The Ticklish Gene
127
tunately, the apatite crystals are each only 500 angstroms long. Now,
the angstrom (named after the Swedish pioneer of spectroscopy Anders
Jonas Angstrom) was invented for measuring atoms and molecules, and
it is not large. Five hundred of them are only one tenth as long as a
single wave of green light. Even today's thinnest microelectrodes are 1
micron (10,000 angstroms) wide, and at the time the thinnest ones
available to us were much larger. It would have been like trying to
measure a grain of rice with a telephone pole.
We would have to do it in a sort of statistical way. Because of the way
bone is built—millions of little scales glued onto larger fibers arranged
in more or less lengthwise spirals along the osteones—I reasoned thus: If
we put an electrode on the lengthwise cut, we'd be contacting mostly
apatite, while an electrode on a face cut across the grain should connect
to a greater proportion of collagen. If that method of electrode hookup
worked and if we had a rectifier in our bones, then we'd be able to pass
current through our samples only in one direction. That was exactly
what happened. Our bone samples weren't as efficient as a commercial
rectifier, but the amount of current we could put through them from a
battery of constant voltage was much greater in one direction than in the
other.
Current flowing "uphill," against the normal flow from P to N semi-
conductors, is called a reverse bias current, and we used it to look for
photoelectric effects. Many semiconductors absorb energy from light,
and any current flowing through the material gets a boost. We arranged
our apparatus so only a small spot of light shone on the bone, because
our silver electrodes were slightly sensitive to light and could produce a
real artifact. With the voltage constant, the light produced an unmis-
takable increase in the current. Now, if bone really contained a rectifier,
the photoelectric effect should be sensitive to the current's direction.
The current in reverse bias should rise more with the same light inten-
sify than the current in forward bias The experiment was simple. We
reversed the battery and turned on the
light. The amperage rose higher
than before. The rectifier was real.