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.[2] 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.

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

" 23³ |5 " | 14,000 | 170 | 82.3 " "

" 24⁴ |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 " "

" 129³ |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.