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.

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TABLE A.
SENSITIVENESS TO LIGHT.
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Selenium | Battery | Resistance in | Resistance in |
cell. | power. | dark. | sunlight. | Ratio.
----------+-----------+---------------+---------------+----------
| | ohms. | ohms. |
No. 22 |5 elements.| 39,000 | 340 |114 to 1
" 23[³] |5 " | 14,000 | 170 | 82.3 " "
" 24[⁴] |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[³] |5 " | 200,000 | 940 |212 " "
" 137 |5 " | 108,000 | 320 |337.5 " "
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Cells which are sensitive to light improve by being used daily, and their sensitiveness becomes less if they are laid aside and not used for a considerable length of time, especially if allowed to become overheated. They should be kept cool, and exposed to light frequently, whether they are used or not.

Mode of measuring cells.—So great is the sensitiveness of these cells to external influences, that it is necessary to adopt some particular system in measuring their resistance and to adhere strictly to that system, as every change in the method of measurement produces a difference in the result, and the different measurements would not be comparable with each other. The reason for this will be explained presently.

The system I have adopted is the Wheatstone's bridge arrangement, with equal sides, never using multipliers except for some experimental purpose. In each multiplier wire I have 500 ohms resistance. When the bridge is balanced, one-half of the current flows through the cell and acts upon the selenium. Between the bridge and the cell is a reversing switch, so that the current can be reversed through the cell without changing its course through the bridge. A Bradley tangent galvanometer is used, employing the coil of 160 ohms resistance. The Leclanche battery is exclusively used in measurements for comparison.

2. The kind of battery employed has a marked effect upon the sensitiveness to light, which is largely reduced or entirely destroyed when the bichromate battery is used. The same cells again become extremely sensitive with the Leclanche battery. We might expect that a change in the current employed would cause a change in the resistance of a cell, but it is not clear how or why it should affect the sensitiveness of selenium to light.

"If one kind of battery current destroys its sensitiveness, may we not suppose that another kind might increase its sensitiveness? Although the Leclanche has operated well, some other may operate still better, and by its special fitness for use on selenium cells may intensify their actions, and so bring to light other properties yet unthought of. Is not here a promising field for experiment, in testing the various forms of battery already known, or even devising some new form especially adapted to the needs and peculiarities of selenium cells?"