Action of Selenium Cells. 217 



tion of high junction resistance. In this respect the form of 

 selenium plate designed by Dr. Werner Siemens, in which 

 the platinum wires form gratings or interlying spirals, is un- 

 questionably the best form to employ when the object in 

 view is to obtain a high sensitiveness to light "*. 



Both authors assume the selenium to possess a special 

 sensitiveness, and this quality to be damaged by the high 

 resistance of the junctions. Therefore they aim at annihi- 

 lating this resistance at the surfaces of contact. Aiming 

 at this annihilation, they enlarge the surfaces of contact and 

 thus diminish the resistance. 



The surfaces of contact should indeed be enlarged. The 

 reasons, however, for which this must be done are, in my 

 opinion, just the opposite to those influencing these two 

 observers. It is the high resistance at the junctions which 

 is the variable, and which is necessary for the microphonic 

 action. The case of this resistance alone being the variable 

 I shall discuss immediately. That the resistance at the 

 junctions is high is a necessary consequence of the imperfect 

 and variable contact. To annihilate it is to render the photo- 

 phone insensitive. And, vice versa, the more extensive the 

 surfaces of contact are, the more sensitive is the selenium cell. 



A further strong support, amongst others, is given to this 

 theory by the observation of Mr. Bidwell f ,. who has made 

 very numerous experiments with selenium cells. " He got the 

 best speech from cells of high total resistance . . . The sele- 

 nium should, however, have a low specific resistance." 



These observations are easily understood by the micro- 

 phonic action of the cells ; for, that the total resistance is 

 to be great and yet the specific resistance of the selenium is to 

 be small, has no other meaning than that there must be a bad 

 contact between good conductors. 



The next question is, if this microphonic action is confined 

 only to the surfaces of contact. I must here recall that 

 Messrs. Draper and Moss J, distinguishing three crystalline 

 modifications of selenium, observe as to their conductivity, 

 " Between these two forms of granular selenium— the appa- 

 rently nonconducting and the comparatively highly conduct- 

 ing — there is another, of intermediate resistance. This modi- 

 fication is highly sensitive to light." 



Just so, according to Eammelsberg § , selenium exists in 

 four allotropic modifications, three of which are crystalline. 



* 1878, Phil. Mag. (5) y. p. 404. 



t London Phys. Soe. Jan. 22, 1881; Tel. Journ. ix, p. 52, 



% 1878, Proc. Irish Ac. (2) i. p. 533, Nov. 10. 



§ 1874, Pogg. Ann, clii. p. 151. 



