MOTION OF GASEOUS IONS IN STRONG ELECTRIC FIELDS 



203 



that the second term is the '' invisible" or random part of the mean 

 energy. Formula (55) thus states that 



random energy _ molecular mass 

 visible energy ion mass 



(56) 



that is, it exhibits in a quantitative way the capacity of storing energy 

 in the form of random motion which light ions travelling in a heavy gas 

 possess; for ions travelling in the parent gas the ordered and the random 

 part of the energy are just equal; for heavy ions in a light gas the dis- 

 ordered fraction becomes negligible. 



There are various ways of understanding the implications of equation 

 (54). One way is to derive the mean energy in a direction at right angles 

 to the field by the use of (53). We find 



{cl} = 



\ aT / 



Mm 



/SM sin^ X + 4m(l — cos x)\/ 1 — cos xV 



(57) 



\ ar /\ ar I 



Now from (54) and (57) the partition of the energy e may be obtained. 



Fig. 10— Order to be followed in computing by recursion the averages 

 (cP^ (cos ??)); case of constant mean free time. 



