SPARKING POTENTIAL IN A UNIFORM ELECTRIC FIELD 61 



which sparking took place when ultra-violet light of 

 small intensity was acting on the negative electrodes. 

 The potentials thus obtained were very definite, and 

 showed no irregularities, being independent of the length 

 of time during which the gas is subjected to the electric 

 force. They were in most cases about two volts below the 

 potential required to spark when no light is acting on 

 the negative electrode. 



The lowering of the sparking potential due to the 

 action of ultra-violet light, which was discovered by 

 Hertz, 1 is more noticeable when the light is strong and 

 the pressure high. According to this theory, it is due to 



the large current proportional to nn Q 



which ensues when X and n are large, before the 

 denominator of the fraction vanishes. The polarization 

 due to the separation of the positive and negative ions dis- 

 turbs the uniformity of the field between the plates in such 

 a way as to facilitate the discharge, as will be explained 

 in section 27. The charge in the gas which causes the 

 polarization is proportional to the intensity of the light, 

 and inversely proportional to the velocity of the ions. 

 This latter quantity diminishes as the pressure of the 

 gas between the plates increases, since X/p for sparking 

 diminishes as the quantity pS rises. It is therefore 

 necessary to use ultra-violet light of very small intensity 

 in order to determine with accuracy the sparking 

 potential which corresponds to the theoretical value 



derived from the equation a (3^ ' =o. 



When no light is falling on the electrodes the sparking 



1 Hertz, Wied Ann., xxxi., 1887. 



