222 On the Microphonic Action of Selenium Cells. 



connected with the galvanometer by means of a switch. And 

 now I found a secondary current which went through the galva- 

 nometer always in the same direction, quite independent of the 

 direction of the primary battery-current. Even when the pri- 

 mary current was produced by a single Leclanche cell only, 

 the deflection by the secondary current was 80 divisions, or 

 about 5 centim. This secondary current is therefore no po- 

 larization-current; for in that case its direction would be 

 always opposite to the primary. But here a change of direc- 

 tion did not occur, the secondary current always flowing from 

 the platinum electrode A to the platinum electrode B, and 

 never in the opposite direction. 



The passage of a current also, like illumination or concus- 

 sion, occasioned a quicker increase of resistance. 



All these qualities, however, are very easily understood; 

 and the very appearance of the piece suggests the explana- 

 tion. There are in it several modifications of the selenium 

 side by side. At the transformation of the one modification 

 into another, as I have pointed out above, heat or electric 

 effect must take place. A state of equilibrium is not reached 

 before that modification has been formed the formation of 

 which is attended by the maximum development of energy — 

 thermal or electric. Such modification again, as also men- 

 tioned above, has a different resistance from the one from 

 which it originated; and therefore the variation of resistance 

 is a necessary consequence of the change of modification. 

 While the transformation goes on, we observe the electric cur- 

 rent; after the transformation is complete we observe the 

 change of resistance. 



Such changes in the modification can be also occasioned by 

 light : a pencil of amorphous selenium assumes, when exposed 

 to the daylight, a grey crystalline surface. 



Two conditions distinguish these changes from the photo- 

 phonic ones, and prove that, as a rule, they do not occur in 

 the photophonic cells. 



On darkening the cell in which only a microphonic action 

 took place, the original state was reached again which it had 

 before being illuminated ; in the piece of selenium wherein 

 a change of modification occurred, this was not the case. This 

 is the reason why the light-spot returns to its original posi- 

 tion on the galvanometer-scale in the case of the selenium 

 cell, but does not in the case of the selenium piece. And, 

 secondly, by such changes of modification a phenomenon is 

 produced which has been called " fatigue." Such fatigue 

 (that is, an insensitiveness) must take place as soon as the store 

 of the one modification which is transforming itself into the 

 other is exhausted. But it cannot take place if we have to 



