218 Dr. J. Moser on the Microphonic 



For evidence of the microphonic action I was at first 

 contented to observe that selenium in this crystalline state, 

 in which it is most brittle and the most powdery, is also 

 most sensitive to light. Afterwards I succeeded in obtain- 

 ing a piece which shows this very clearly. Between two 

 platinum wires of 3 centim. length was a selenium plate 8 

 millim. in width and hardly 1 millim. in thickness. Heated 

 in air and annealed, the whole surface had taken the light 

 grey colour of a crystalline state; half the upper surface, on 

 the side of the one platinum wire, was smooth and continuous; 

 the other half, near the other platinum wire, was powdery and 

 granular. Alternately one of the two halves was kept con- 

 stantly dark, and the other half at the same time intermittingly 

 illuminated. Thus I could observe that almost the total 

 sensitiveness of the whole plate resided in the granular coarse 

 half of the surface. This shows that microphonic effects occur 

 also in the selenium itself. But even if the selenium were per- 

 fectly homogeneous, and remained so during the illumination, 

 the rays would produce heat and change of volume, which 

 would have a microphonic influence on the contact- 

 By such microphonic changes the efficiency of the sele- 

 nium celj would be explained, and thereby the selenium would 

 be removed from its isolated position and coordinated with all 

 the other bodies in which change of volume and of form 

 under the influence of radiation have been observed. 



I, however, do not deny that light may have other effects 



on the selenium. That such is the case is indeed one of the 



causes which have hitherto masked the microphonic action of 



the cells. That my present view of the simple action of the 



selenium did not present itself at once is owing to two causes. 



In the first place, there is the fact that the microphonic 



1 variations or changes in volume and in form, attributed com- 



/ monly to the so-called heat-rays, are produced in the selenium 



by the illuminating rays. 



In the second place, complication is introduced by the fact 

 that selenium exists in four allotropic modifications, and that 

 light is able to exert on these divers influences. Though 

 these are not essential to the action of the photophone, it has 

 nevertheless been assumed by others, and at first by myself, 

 that such changes might be the key 'for the explanation of 

 the selenium photophone. 



Of these two impediments which stood in the way of the 

 perception of the microphonic action of the cells, I intend to 

 speak more in detail. And first a few words on what, for the 

 sake of brevity, may be called the heating effect of the light- 

 rays. 



