﻿582 Mr. Karl T. Oompton on Contact Difference of 



V = a / , where e and m are the charge and mass 



V m & 



associated with an electron, and V is the largest potential 



against which it can reach C from D. 



The general plan of procedure was to (1) polish the 

 plate D with chalk-dust; (2) measure the contact difference 

 of potential between D and C ; (3) exhaust the apparatus, 

 first with the mercury pump and then with coconut char- 

 coal immersed in liquid air, and then take readings for a 

 " distribution of velocity " curve by measuring the rate of 

 deflexion of the electrometer for different values of V as 

 described ; (4) let air into the apparatus and again measure 

 the contact difference of potential. The average of the two 

 values for the contact difference of potential was the value 

 made use of. These two readings were made since, after 

 polishing, metals become more electronegative with time. 

 This is especially true in the case of aluminium *. Then a 

 perforated box C of another metal was substituted and the 

 previous procedure repeated, keeping all other conditions in 

 the two cases as nearly identical as possible. If there is an 

 effective field between and D due to the contact difference 

 of potential between C and D, the curve obtained in the 

 second case should differ from that in the first case by being- 

 shifted along the axis of V by an amount corresponding to 

 the contact difference of potential between the metals used 

 for in the two cases. 



The contact difference of potential between C and D was 

 measured by the following method (fig. 3). C and D were 



Fi*. 3. 



placed in the aluminium-lined cavity in a lead block. The 

 air between C and D was ionized by radium placed at R. 

 If the lead block and box C are earthed, an electrometer, 

 connected with D, will charge up to a potential equal to the 

 contact difference of potential between C and D. In this 



* Winkelmami, Handbuch der Physik, 1st edition, vol. iii. p. 118. 



