lONIZATION BY NEGATIVE IONS 21 



**-- 



10. Properties of the curves - = f(- J . 



The properties of the curves are in general agree- 

 ment with the indications of the theory. For simplicity 

 the pressure may be taken as unity which has been fixed 

 as the pressure due to 1 millimetre of mercury. The 



*y 



zoo 



$00 



4OO 



500 



X+p. 



(X in volts per centimetre, p in millimetres of mercury.) 



Figure 8. 



co-ordinates of any point then represent the electric 

 force X and the number a of molecules of the gas at a 

 millimetre pressure that are ionized by a single ion in 

 moving through one centimetre under this force. For 

 small forces the quantity a practically vanishes; the 

 ions in this case do not acquire sufficient velocity along 

 any of their free paths to generate others by collisions. 



