230 h'arss — Ionization hy Collision in (rases and Vapors. 



This effect is greatly increased by increasing the electromotive 

 force and hence the abrupt rise in the third stage of the cur- 

 rent is explained. Townsend has also shown that if the applied 

 electromotive force is not too great, the negative ion alone is 

 effective in producing new ions ; hut as the applied force is 

 increased and the sparking potential is approached, the positive 

 ions also acquire the property of producing others to an appre- 

 ciable extent. 



When the initial ionization consists of positive and negative 

 ions uniformly generated in a gas between parallel-plate elec- 

 trodes by the action of Rontgen rays, Townsend has derived 

 the following formula : 



c = c 



aL 



where C is the constant current when no new ions are pro- 

 duced by collision (second stage), C the current corresponding 

 to any field strength X after collision has begun (third stage), 

 L the distance between the plates, e Napier's base, and a the 

 number of ions produced by each negative ion by collisions 

 with gaseous molecules in passing through a centimeter of the 

 gas. C, C and L may be determined experimentally, and there- 

 fore a may be calculated. The accuracy of this formula has 

 been tested and it has been found that for a given field strength 

 and a constant pressure, the value of C/C varies with L, but 

 that the values of a are independent of L within the limits of 

 experimental error. 



When the pressure is constant, a increases with the field 

 strength and approaches a maximum value, which is reached 

 when the field strength is great enough to produce a new pair 

 of ions at each collision. If the pressure is reduced, the field 

 strength being constant, a increases to a maximum and finally 

 diminishes again. This agrees with the theory, for at a high 

 pressure the free paths of the ions are very short and the ions 

 do not acquire a sufficient velocity to ionize by collision. As 

 the pressure is reduced the free paths are lengthened and the 

 proportion of collisions resulting in new ions is increased. 

 When the pressure is reduced beyond a certain critical point, 

 the value of a diminishes, for then the number of molecules of 

 gas becomes less and therefore the number of ions produced by 

 collision becomes smaller in the same proportion. 



Townsend has also shown that 



a/p =f(X/p) 



where p is the gaseous pressure and X the field strength ; the 

 function f depending upon the nature of the gas. Experi- 

 ments verify this equation very accurately for all the gases that 

 have been employed. 



