JONIZATION BY NEGATIVE IONS 



19 



is determined by the force X and the length of the path 

 which is terminated by the collision. The lengths of the 

 free paths are inversely proportional to the pressure, so 

 that if the pressure is increased from p to zp all the 

 free paths will be reduced to l/z of their original value. 

 If the force X remained unaltered, the velocities on 



JOO SCO 3CO 4OO SCO 600 



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

 Figure 6. 



collision would be reduced, but if the force is increased 

 to #X the velocities will be restored to their original 

 values and the number of ions arising from a given 

 number of collisions will be the same as before. Since 

 the total number of collisions per centimetre is increased 

 in the same ratio as the pressure, the value of a will 

 be increased to za, when X and p become ^X and 



c 2 



