MOTION OF GASEOUS IONS IN STRONG ELECTRIC FIELDS 



189 



and 



D = const -r 



kT 

 M 



(29c) 



The information in the formulas (29) is now new, but dependent on (27) 

 through a universal relation first discovered by Nernst and derived 

 independently for gases by J. J. Thomson; it is widely known as the 

 Einstein relation. It states that 



da m 



(30) 



Equation (30) contains of course more than is obtainable from (27) 

 and (29), since it relates one undetermined constant to another in a 

 known way. 



The dimensional methods of this section are convenient for a rough 

 classification of experimental material. Figs. 3 to 7 show the drift velocities 



100 



xiO'* 



10 40 



30 



20 



15 



W 



a^: 



\r- 





.■■X 



-^^ 



?" 



HELIUM 



PRESSURE 



IN MM Hg 



1.675 



3.5 2 



8.60 



12.72 



22.2 



>^ 



30 40 

 ACCELERATION 



80 100 



150 



200 



X 10"* 



NUMBER DENSITY 



Fig, 3 — Drift velocity in an electric field of He+ ions in helium gas. Compari- 

 son of observed results with an "asymptotic" straight line of slope \i. 



