[Footnote: Paper read before the American Association for the Advancement
of Science, at Philadelphia, Sept, 5, 1884.]
By C.E. FRITTS, 42 Nassau St., New York, N.Y.
In all previous cells, so far as I am aware, the two portions or parts of
the selenium at which the current enters and leaves it have been in
substantially the same electrical state or condition. Furthermore, the
paths of the current and of the light have been transverse to each other,
so that the two forces partially neutralize each other in their action
upon the selenium. Lastly, the current flows through not only the surface
layer, which is acted upon by the light, but also the portion which is
underneath, and not affected thereby, and which therefore detracts from
the actual effect of the light upon the selenium at the surface.
My form of cell is a radical departure from all previous methods of
employing selenium, in all of these respects. In the first place, I form
the selenium in very thin plates, and polarize them, so that the
opposite faces have different electrical states or properties. This I do
by melting it upon a plate of metal with which it will form a chemical
combination, sufficient, at least, to cause the selenium to adhere and
make a good electrical connection with it. The other surface of the
selenium is not so united or combined, but is left in a free state, and a
conductor is subsequently applied over it by simple contact or pressure.
During the process of melting and crystallizing, the selenium is
compressed between the metal plate upon which it is melted and another
plate of steel or other substance with which it will not combine. Thus by
the simultaneous application and action of heat, pressure, chemical
affinity, and crystallization, it is formed into a sheet of granular
selenium, uniformly polarized throughout, and having its two surfaces in
opposite phases as regards its molecular arrangement. The non-adherent
plate being removed after the cell has become cool, I then cover that
surface with a _transparent conductor of electricity_, which may be a
thin film of gold leaf. Platinum, silver, or other suitable material may
also be employed. The whole surface of the selenium is therefore covered
with a good electrical conductor, yet is practically bare to the light,
which passes through the conductor to the selenium underneath.[5] My
standard size of cell has about two by two and a half inches of surface,
with a thickness of 1/1000 to 5/1000 inch of selenium. But the cells can,
of course, be made of any size or form. A great advantage of this
arrangement consists in the fact that it enables me to apply the current
and the light to the selenium in the same plane or general direction,
instead of transversely to each other as heretofore done, so that I can
cause the two influences to either coincide in direction and action, or
to act upon opposite faces of the selenium and oppose each other,
according to the effect desired.
[Footnote 5: The method of constructing the cells was described in the
SCIENTIFIC AMERICAN SUPPLEMENT, No. 462, for Nov. 8, 1884, page 7371.]
By virtue of the process and arrangement described, my cells have a
number of remarkable properties, among which are the following:
1. _Their sensitiveness to light_ is much greater than ever before known.
The most sensitive cell ever produced, previous to my investigations, was
one made by Dr. Werner Siemens, which was 14.8 times as conductive in
sunlight as in dark. In table A, I give results obtained from a number of
my cells.
It will be observed that I have produced one cell which was 337.5 times
as conductive in hazy sunlight as in dark. The tremendous change of
resistance involved in the expression "337.5 times" may perhaps be more
fully realized by saying that 99.704 _per cent_. of the resistance had
disappeared temporarily, under the joint action of light and electricity,
so that there remained _less than 3/10 of 1 per cent_. of the original
resistance of the selenium in dark.
In order to obtain these high results, the cells must be protected from
light when not in use. The resistance is first measured while the cell is
still in total darkness. It is then exposed to sunlight and again
measured. It is also necessary to send the current in at the gold
electrode or face, as the cell is much less sensitive to light when the
light acts upon one surface of the selenium and the current enters at the
opposite surface. When the two influences, the light and the current, act
through the gold, in conjunction, their forces are united; and, as every
atom of the selenium is affected by the light, owing to the extreme
thinness of the plate, we have the full effect shown in the measurements.