he 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.

TABLE A.

SENSITIVENESS TO LIGHT.

-----------------------------------------------------------------
Selenium | Battery | Resistance in | Resistance in |
cell. | power. | dark. | sunlight. | Ratio.
----------+-----------+---------------+---------------+----------
| | ohms. | ohms. |
No. 22 |5 elements.| 39,000 | 340 |114 to 1
" 23[6]|5 " | 14,000 | 170 | 82.3 " "
" 24[7]|5 " | 648,000 | 2,400 |270 " "
" 25 |5 " | 180,000 | 930 |196.5 " "
" 26 |5 " |