﻿Potential and the Velocities of Electrons. 581 



but in this case- the foil was much thinner than in the others. 

 Then there was one of platinum wire gauze and one of nickel 

 wire gauze, but these were not of the same size wire nor of 

 equal mesh. The whole apparatus was enclosed in an air- 

 tight cylindrical brass box ST which was connected with a 

 mercury pump and a tube of coconut charcoal. 



The electrons leave at b on the plate D. The relative 

 number reaching the perforated box C is regulated by 

 varying the potential between C and D by means of the 

 sliding resistance R. The voltmeter V shows this difference 

 of potential. The electrons which reach G are caught by the 

 strong field between C and B and are swept on to the brass 

 box B which is connected to a Dolazalek electrometer with 

 a sensibility of 420 divisions per volt. The connexions are 

 so arranged that initially B is at zero potential in order to 

 avoid, as far as possible, any leak of electricity from B. 

 The perforated box C serves a triple purpose. It prevents 

 the loss of electrons from B due to diffusely reflected light ; 

 it precludes the possibility of reflected electrons * ; and it 

 enables the potential V to be kept constant while the electro- 

 meter is charging up. Finally, everything connected with 

 the electrometer is enclosed in earthed metal casing to pre- 

 vent electrostatic disturbances from external causes. 



3. Method. — In taking the readings for the " distribution 

 of velocity " curve, a difficulty arose from the flickering of 

 the iron arc, which caused large variations in the intensity 

 of the light during the time necessary to take a complete set 

 of readings. This was overcome by the method of taking- 

 reading illustrated in Table I. Here V represents the 

 reading of the voltmeter and A is the deflexion of the electro- 

 meter in 15 seconds. Readings were taken alternately with 



V = and V= whatever number of volts was desired. Then 

 the ratio of the deflexion for Y-— (?) to the deflexion for 

 V=0 was determined and placed in the column marked R. 

 Thus all the readings were referred to the readings with 



V = as a standard. In plotting, the average of a number 

 of these ratios for a given value of V was plotted as an 

 ordinate. The maximum average ratio obtained with any 

 value of V was taken as unity and the other ratios plotted 

 accordingly. Thus the ordinates are proportional to the 

 number of electrons reaching B in a unit time. The 

 voltmeter readings w r ere plotted as abscissa?, being called 

 negative when in such a direction as to hinder an electron 

 passing from I) to C. The velocity of an electron is then 



* 0. v. Baeyer, Yerh. d. D. Phys. Ges. x. p. 96 (1908). 



