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IV. The Electrical Resistance of Selenium Cells. 

 By Shelford Bid well, M.A., LL.B* 



IN June 1881 a paper was read before the Physical Society 

 by Dr. Moser, on " the Microphonic Action of Selenium 

 Cells." In this paper a very ingenious attempt was made to 

 show that the effect of light in reducing the electrical resist- 

 ance of selenium might be accounted for on perfectly well 

 understood principles, without assuming the existence in the 

 case of this substance of some law or property sui generis and 

 hitherto unobserved. 



Dr. Moser's theory is shortly as follows : — There is always 

 imperfect contact between the metallic electrodes and the 

 selenium which together constitute a so-called " selenium 

 cell." Selenium reflects the invisible portions of the spec- 

 trum, absorbing principally the visible or illuminating rays : 

 the vibrations thus taken up assume the form of heat; and the 

 temperature of the selenium cell is thereby raisedf. In con- 

 sequence of this rise of temperature the selenium expands ; it 

 is accordingly pressed into closer contact with the electrodes, 

 and, as in the case of the microphone, the resistance of the 

 system is proportionately diminished. When the cell is 

 screened from the light, the absorbed heat is quickly radiated 

 away ; the selenium contracts to its former volume, and the 

 original degree of resistance is restored. Thus, according to 

 Dr. Moser's paper, the whole mystery is easily and completely 

 explained. 



This theory can evidently be submitted to a very simple 

 and conclusive test. If it is true that the observed effects are 

 due merely to a rise of temperature, then it is clearly imma- 

 terial whether such rise of temperature is brought about by 

 the heating action of light or by the direct application of heat 

 in the ordinary way. Instead of exposing a selenium cell to 

 the light, let it be enclosed in a dark box and warmed over a 

 gas-burner ; then, if the theory be correct, the resistance of 

 the cell should at once begin to fall, This, however, is not 

 found to be the case. I have in my possession a number of 

 selenium cells the resistance of which is immediately dimi- 

 nished by the smallest accession of light ; but in the case of 

 all of them (except one, of which I shall say more presently) 

 the immediate effect of the direct application of heat is not a 

 a fall, but a rise in the resistance. When the temperature of 

 the cell reaches a point which is in general a few degrees 



* Conxnmnicated by the Physical Society, having been read at the 

 Meeting on November 25, 1882. 



f " Selenium" Dr. Moser says, " is heated by light." 



