68 Dr. Tate and Dr. Foote on Resonance and Ionization 



centre o£ the hot wire. The curves of fig. 7 for potassium 

 were obtained with the potential applied at the positive end 

 of the hot wire, while those of fig. 8 were obtained with the 

 potential applied at the negative end. It will be seen that 

 the initial velocity of the electrons correcting for the drop in 

 potential along the hot wire must have been 1*6 volts, a sur- 

 prisingly high value. This is interesting in connexion with 

 the recent experiments of Wood and Okano*, who found 

 that an applied potential of 0*5 volt was sufficient to excite 

 the D-lines in sodium vapour, indicating, if the resonance 

 potential exciting the D-line in sodium vapour is taken as 

 2'1 volts, that the initial velocity of the electrons in their 

 experiments must have been 1*6 volts. It is our opinion 

 that the appearance of the many-lined spectrum in metallic 

 vapours at potentials lower than the ionizing potential is due 

 to the presence of high-speed electrons rather than to a 

 decrease in the value of the critical potentials. In no case 

 was there any evidence of a decrease in the value of the 

 resonance potential even under conditions which allowed the 

 striking of the visible arc at applied potentials considerably 

 lower than the ionizing potential. 



The curves obtained for zinc, figs. 9 and 10, deserve special 

 attention . It will be noted that the points a to g form a 

 double series of points, <x, c, e,g, and 6, d,f, having a common 

 difference in potential of 4*1 volts, This common difference 

 is taken as the value of the resonance potential. An expla- 

 nation for the double series of points is lacking, however. 

 It is possible that there are two groups of electrons pos- 

 sessing differing initial velocities, or that there is a se- 

 condary resonance potential at 2 '3 volts. The fact that 

 there is no evidence of a succession of points differing in 

 potential by 2*3 volts would indicate that the first explanation 

 was the correct one. A consideration of the total current 

 curves, 7 and 8, fig. 10, however, leaves the question very 

 much in doubt. The total current shows a rapid falling off 

 in rate of increase at the points a . . . e for which no 

 satisfactory explanation has been found. At all events, 

 however, the points P indicating ionization of the zinc 

 vapour are very definitely located at an effective potential 

 of 9*5 volts. The fact that there are not two such points P 

 on each curve indicates that there are not present two groups 

 of electrons having different initial velocities. Further work 

 is being carried out on zinc vapour with a view to clearing 

 up some of these difficulties. 



* Wood and Okano, Phil. Mag. xxxiv. p. 177 (1917). 



