therefore raise the resistance, by sending an intermittent
or an alternating current though the cells, and in their new condition
they at once become extremely sensitive to light, currents, and other
influences. In some cases they drop to the low state again, and require
to be again brought up, until, after a number of such treatments, they
remain in the sensitive state. Occasionally a cell will persist in
remaining in the insensitive state. The before mentioned treatment raises
it up for a moment, but, before the bridge can be balanced and the
resistance measured, it again drops into the low or insensitive state.
Some cells have been thus stimulated into the high or sensitive state
repeatedly, and every means used to make them stay there, but without
avail; and they have had to be laid aside as intractable.

In the earlier stages of my investigations, before the discovery of this
dual state and the method of changing a cell from the insensitive to the
sensitive condition, hundreds of cells were made, finished, and tested,
only to be then ruthlessly destroyed and melted over, under the
impression that they were worthless. Now, I consider nothing worthless,
but expect sooner or later to make every cell useful for one purpose or
another.

The most singular part of this phenomenon is the wide difference in the
resistance of the cells in the two states. In the low state, it may be a
few ohms, or even a few hundredths of an ohm. In the high state, it is
the normal working resistance of the cell, usually between 5,000 and
200,000 ohms, but is often up among the millions. The spectacle of a
little selenium being stimulated, by a few interruptions of the current
through it, into changing its resistance from a fraction of an ohm up to
a million or several millions of ohms, and repeatedly and instantly
changing back and forth, up and down, through such a wide range, we might
almost say changing from zero to infinity, and the reverse, instantly, is
one which suggests some very far-reaching inquiries to the electrician
and the physicist. What is the nature of electrical conductivity or
resistance, and how is it so greatly and so suddenly changed?

6. _Radio-electric current generators_.--My cells can be so treated that
will generate a current by simple exposure to light or heat. The light,
for instance, passes through the gold and acts upon its junction with
the selenium, developing an electromotive force which results in a
curr