﻿Potential and the Velocities oj Electrons. 



587 



influence of the contact difference of potential, and (2) the 

 change of contact difference of potential which all metals, 

 and especially the electropositive metals, undergo with 

 time. 



6. The Velocities of the Electrons emitted by liferent 

 Metals. — Several physicists * have investigated the velocities 

 of the electrons emitted by different metals and have arrived 

 at conflicting conclusions. 0. v. Baeyer and A. Gehrts 

 found values for the maximum initial velocity of the electrons 

 from aluminium, copper, and gold to be that velocity which 

 w r ould give the electrons sufficient initial kinetic energy to 

 carry them against a difference of potential of 6*3 volts. 

 They concluded that the maximum initial velocity for these 

 three metals is the same. E. Ladenburg found 1*76 volts 

 for platinum, 1*55 volts for copper, and 1*07 volts for zinc, 

 for A, = 2 10 fifx. Millikan and Winchester, on the other 

 hand, found the values given in Table III., at 26° C. 



Table III. 



Metal. 



Velocity. 



Metal. 



Velocity. 



Mg 



•840 

 •738 

 •197 

 •000 

 •394 



m ... 



1-126 

 1-215 

 1-174 

 1-135 

 1-215 

 1-340 



Al 



Fe 



Zn 



Pb 



Brass 



Cu 



Sb 



Au 





Aff 





In view of the results of the preceding section it is evident 

 that unless a correction is made for the contact difference of 

 potential, an electropositive metal will give off electrons 

 with apparently smaller velocities than those leaving an 

 electronegative plate, even though they may actually possess 

 equal velocities. The experiments described in this section 

 were performed to see whether, when the contact difference 

 of potential was corrected for, it might not be found that all 

 metals give off electrons with equal velocities, under normal 

 conditions. 



In Table III. the elements have been arranged in the order 

 of the usual electro-positive series ; and it is evident from 



* P. Lenard, Ami. d. Pki/s. p. 149 (1902) ; E. Ladenburg, Verh. d. B. 

 Phys. Ges. ix. p. 503 (1907) ; Millikan & Winchester, Phil. Mag. xiv. 

 p. 194 (1907); O. v. Baeyer & A. Gehrts, Bet. d. J). Phys. Ges. xxl. 



p. 870(1910). 



