254 THE ROYAL SOCIETY OF CANADA 



observed in the photo-electric current need not be a property of the 

 illuminated crystal, but may be due to the condition of the dark 

 current. On illumination in this irregular condition, it was sometimes 

 found that the photo-electric current flowed in a direction opposite 

 to the normal one, indicating either a photo-negative effect or the 

 presence in the crystal of counter fields greater in magnitude than the 

 external field. The latter hypothesis appears more natural. 



In the results recorded above, there is a clear relation between 

 dark and photo-electric currents^ — ^large photo-electric current in the 

 direction of large dark current, and irregularity in photo-electric 

 current when there was irregularity in the dark current. It was also 

 found that the diamonds which were not photo-sensitive were ab- 

 solutely non-conducting in fields as high as 11,500 volts per cm. This 

 indicates that the presence of a dark current is necessary for the 

 existence of a photo-electric current. That this conclusion does not 

 hold generally has been shown in work with some substances at very 

 low temperatures, when the dark current almost vanished while the 

 photo-sensitivity increased. It is usually true, however, that the 

 magnitude of the photo-electric current depends on that of the dark 

 current through the crystal. 



(/) Counter E.M.F. in Crystal 



In discussing photo-electric current, mention has been made of 

 the analogy between the irregular secondary current of Gudden and 

 Pohl, and the irregularity found with diamond on high voltage. It 

 has also been pointed out that illumination of the crystal, when the 

 dark current exhibited irregularity, sometimes resulted in an ap- 

 parently photo-negative efïect. In this connection it is interesting 

 to point out the conclusions reached by Coblentz^^ in his work on the 

 photo-negative properties of stibnite and molybdenite. Whereas 

 Gudden and Pohl analyse the photo-electric current into two com- 

 ponents:- — the Primary, which is instantaneously established and 

 prevails on low voltage, and the Secondary, which takes time to grow 

 and prevails on high voltage- — Coblentz comes to the conclusion that 

 there are two contending forces acting Firstly, the one which causes 

 photo-positive response acts quickly, prevails on low voltage and is 

 similar to a resistance decrease; secondly, the one which causes 

 photo-negative reaction grows slowly, is predominant on high voltage, 

 and corresponds to the building up of a counter electromotive force. 

 The analogy is striking between the photo-positive reaction and the 



"W. W. Coblentz, Sci. Papers Bur. of Stands, No. 398, p. 624. 



