On the Photo-electric Effect of Ultra-Violet Light. 297 



velocity the carrier would acquire under the electric field. 

 Such a calculation will show that the positive ions cannot be 

 aggregates condensed round a charge, but that, on the 

 contrary, to give so high a velocity the mass of the carrier 

 cannot be much, if at all, greater than the mass of an atom 

 of hydrogen ; in other words, that the mass of the carriers 

 in the flame are comparable with those found in the 

 Canalstrahlen. I regret that I overlooked the mention by 

 Professor Wien in his paper of rays deflected in the opposite 

 direction to the Canalstrahlen. 



I am, Gentlemen, 



Yours very sincerely, 



J. J. Thomson. 



XXIV. The Relation between the Intensity of the Ultra- Violet 



XJtuiib /tioutiLi/ uil u, xt truwuiueoy vioiai ucu, z^liil, jl butt: mini int 



Quantity of Electricity which is set free from the Surface, 

 By I. 0. Griffith, M.A., Fellow of St. Johns College, 



Oxford * 



IT has generally been assumed that the amount of electricity 

 emitted by a negatively charged zinc plate under the 

 action of ultra-violet light is proportional to the intensity of 

 the light. This, however, does not seem to be in harmony 

 with the more recent developments of the subject, and as no 

 very accurate experiments have been made to find the con- 

 nexion between these quantities, it was thought desirable to 

 make a careful investigation over a wide range of light- 

 intensity. 



The experiments here described were made chiefly with 

 the object of investigating this point, and the results show 

 that if I denote the intensity of the light, E the corresponding 

 photo-electric effect, then E/I is not constant but increases 

 with increasing light-intensity. 



As it is desirable that j should vary over a large range, 

 and as the quantity of electricity set free from the zinc 

 plate is very small for the lower values of I, an apparatus 

 was constructed by means of which the effect is multiplied 

 by a constant factor of a suitable order of magnitude. In 

 what follows, the part of the apparatus containing the plate 

 will be referred to as the detector. It is shown diagram- 

 matically in fig. 1, and consists essentially of an ebonite 

 chamber which can be exhausted of air, and two parallel zinc 



* Communicated by Prof. J. S. Townsend, F.R.S. 

 Phil. Mag. S. 6. Vol. 14. No. 80. Aug. 1907. X 



