Section III, 1922, 



[241] 



Trans. R.S.C. 



On PJwto-electric Conductivity of Diamond and Other Fluorescent 



Crystals 



By Miss M. Levi, B.A. 



(Read May Meeting, 1922) 



1. Introduction. 



A number of substances have been discovered which, like selenium, 

 exhibit the phenomenon of photo-electric conductivity, — that is, their 

 electrical resistance undergoes a change on exposure to radiation. 

 In most cases the resistance is decreased on illumination, when the 

 effect is said to be photo-positive. In some cases, however, it has 

 been found that under certain conditions of applied voltage and 

 wave-length of exciting light, the resistance is increased. The latter 

 effect is said to be photo-negative. In no case has a substance been 

 found whose resistance is always increased when light falls on it. 



Among the substances which show the effect are minerals like 

 molybdenite, stibnite and silver sulphide, which are to a greater or 

 less extent conductors of electricity, and others such as zinc sulphide, 

 cinnabar and diamond, which are known as insulators, but which, 

 nevertheless, become conducting under the influence of light. 



In view of the fact that a number of fluorescent materials are 

 photo-sensitive, it was thought to be of interest to investigate the 

 photo-electric properties of some fluorescent crystals. Six diamonds 

 and a few samples of fluorescent kunzite and willemite were chosen 

 for investigation. 



2. Apparatus and Method. 



It was first desired to obtain the relation between the photo- 

 electric current per unit of exciting light energy, and the wave length 

 of exciting light. For this purpose the following apparatus was used. 



The source of radiation was a mercury arc lamp in quartz, of 

 the Herœus type, which was run at a steady current of 2 amperes. 

 The lamp was mounted in front of the slit of a Hilger constant devia- 

 tion spectrometer, fitted with quartz prism and fluorite lenses, so 

 that radiation far down in the ultra-violet could be used. For work 

 with the continuous spectrum, a powerful self-regulating carbon arc 

 was employed. 



16— c 



