﻿544 Dr. J. Robinson on the Photoelectric 



LMN at a low pressure. A wire passing through the 

 tube L was made to touch the walls so that the potential 

 of the latter could be controlled. The tube M was used 

 for connecting to the mercury -p amp, and through N a wire 

 protruded a short distance into the tube K. This wire was 

 used as anode when the current was passed from the 

 platinum electrode Gr as cathode to deposit a film on the 

 quartz plate A. 



Before this was done, Gr was kept as far as possible away 

 from A, and the current passed for a long time to drive 

 out some of the occluded gas. Then Gr was moved up to 

 about 1 cm. from A and the film deposited. 



The rod B was connected in the ordinary way to one pair of 

 quadrants of a Dolezalek electrometer. In making measure- 

 ments of the velocities of the electrons, all stray wires, L, M, 

 and the platinum electrode Gr were earthed, so that the thin 

 film was in an earth-connected cylinder. The maximum 

 potential taken up by the film under the action of ultra- 

 violet light was measured, when it was turned to face the 

 light (incident), and when it was turned away from the 

 light (emergent). The velocity v of an electron is connected 

 with this maximum potential V by the formula ^mv 2 = eY. 



Scales were pasted around the two portions of the ground 

 joint so as to show exactly what position the quartz plate 

 occupied. A quartz mercury lamp was used as source of 

 light. 



The measurements of the photoelectric current were made 

 by shunting the pairs of quadrants of the electrometer by a 

 high resistance. This resistance was made by depositing 

 platinum on glass. It was found that if the vessel in which 

 this was done was pumped out as far as possible and sealed 

 off, a very constant high resistance was obtained, which is 

 very suitable for the present purpose. The currents were 

 measured for different fields between the film and the walls 

 of the vessel, which fields were obtained by charging the 

 walls to different potentials. 



The relative thicknesses of the films are given in terms of 

 the lengths of time taken to deposit them. 



The last column gives the ratio of the emergent velocity 

 to the incident velocity. It will be noticed that for the films 

 40 minutes and 50 minutes, different values are given for 

 the actual velocities. These different values for the same 

 film were obtained under different conditions. The measure- 

 ments were always made at the highest vacuum it was 

 possible to obtain, but on a few occasions it was necessary 

 to let air into the tube. On pumping out again, it was 



