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 " " ----------------------------------------------------------------- [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.] [Footnote 7: No. 24 was measured with a bridge multiplier of 6 to 1.] 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.