&*)6 Mr. Otto Stuhlmann on Difference in Photoelectric 



axis of two brass cylinders placed horizontally end to end. 

 These were insulated from their supports by quartz stays. 

 In the centre of each cylinder perpendicular to the ray, a 

 quartz plate one millimetre in thickness was suspended and 

 was covered on one of its faces with a thin film of the metal 

 under investigation, obtained b}^ sputtering from a cathode in 

 ■vacuo. The films were maintained at a constant negative 

 potential of 200 volts. Each cylinder was connected to 

 one set of quadrants of a Dolezalek electrometer giving 

 716 divisions deflexion per volt at a scale distance of one 

 metre, with a constant potential of 80 volts on the neadle. 



It may be of interest to add that a phosphor-bronze strip 

 suspension was used in the electrometer. Any desired sensi- 

 tiveness could be obtained by gradually reducing the breadth 

 and thickness of the strip by dissolving it away with dilute, 

 warm nitric acid. 



The tandem arrangement of mounting the plates (see fig. 1) 

 allowed one to use a differential method. It consisted in 

 measuring for various thicknesses of metal films the saturation 

 current for two successive positions of the plates. 



Ficr. 1. 



LIGHT. 



M 1 — Earth 



U 8. 



®—J 



The plate A was always maintained stationary with the 

 film side facing the light. The second plate could be rotated 

 about a vertical axis so that the film successively faced 

 towards the light (B 2 ) and away from the light (Bj). 



t> 



In the first case the ratio of the ionization — -was measured, 



A 



giving the ratio of the emergent effect at B to the incident 



7} 



effect at A. In the second case the ratio -£ was obtained, 



A 



giving the ratio of the incident effects for the two plates. 

 Hence the ratio ~ to -— gave the value of the ratio of the 

 emergent effect to the incident effect at the same plate B. 



