716 



Prof. H. A. Wilson on the Velocity of the 



using a P.D. of! 5'60 volts. With this P.D., the first term 

 Sir J. J. Thomson's equation can be safely neglected. 



m 



Metal. 



Current. 



Caesium . 



. 123 



Rubidium 



. 41-4 



Potassium 



. . 21 



Sodium . 



3-5 



Lithium . 



. . 1-3 



In order to use these values of the currents to calculate 

 the /'s we require another relation. If the positive ions 

 consist of single atoms, then their velocities ought to be 

 approximately inversely proportional to the square roots of 

 their atomic weights (M), consequently / ought to be pro- 

 portional to <v/M. 



Instead of (5) we can write 



Aft 



here A is a constant. 





Putting /'=B \/M this becomes 



ABiM i 



(l-BMi)r 



Two values of i and M then suffice to determine A and B. 

 Using the values for caesium and sodium gives B = 0*08594 

 and A = 3*67. 



"With these values of A and B we get the values of f 

 given in the second column of the following table : — 



Metal. 

 Caesium 



0-99 

 079 

 0-54 

 041 

 0-23 



099 

 0-96 

 91 

 041 

 021 



71 



89 



130 



170 

 305 



l\. 



71 



73 



77 



170 



333 



Rubidium 



Potassium 



Sodium 



Lithium 



The column headed f contains the values of f required by 

 the observed currents. The differences between / and /' are 

 not very great, except in the case of potassium. It seems, 

 therefore, that the assumption that/ varies as -v^M is roughly 

 true. The column headed k l contains the values of the 

 velocities of the positive ions got by using the numbers for 

 /, and that headed k x ' those corresponding to the numbers 



