[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     "    |   135,000     |    710        |190    " "
" 107   |5     "    |   118,000     |    740        |159    " "
" 108   |5     "    |   200,000     |    900        |222    " "
" 122   |5     "    |    56,000     |    220        |254.5  " "
" 129[6]|5     "    |   200,000     |    940        |212    " "
" 137   |5     "    |   108,000     |    320        |337.5  " "
------------------------------------------------------------

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[Footnote 6:  Cells No. 23 and No. 129 are now in possession of Prof.  W. Gryllis Adams, of King’s College, London; Dr. Werner Siemens has No. 25, and Prof.  George F. Barker, of Philadelphia, has No. 26.]