10 THE THEORY OF IONIZATION OF GASES 



the values obtained from the curves, figure 4, being 2*8, 

 2'9, and 2*9. Retaining the same force and making further 

 reductions in the pressure, a is found to diminish ; its 

 mean value deduced from similar experiments at '171 

 millimetre pressure being 2*15. 



When the pressure is constant, a increases with the 

 force and approaches a maximum value, which is 

 attained when the force is so great that a new pair of 

 ions are formed at each collision. For the smaller forces 

 a has a smaller value since the velocity acquired by an 

 ion along a free path is only large enough to produce 

 others by collision in the case of the longer free 

 paths. 



When the force is constant and the pressure reduced, 

 as in the experiments at 160 volts per centimetre which 

 have been quoted, the value of a increases, attains a 

 maximum, and finally diminishes again. This also 

 agrees with the theory, for at high pressures the free 

 paths are very short and the ions do not acquire a large 

 velocity and do not ionize the molecules with which they 

 collide. As the pressure is reduced the free paths are 

 increased, so that along the longer paths a high velocity 

 is attained. The proportion of collisions which result 

 in new ions being formed thus increases, but the total 

 number of collisions per centimetre diminishes. When 

 the pressure is reduced beyond a certain point the 

 values of a begin to diminish, as the number of molecules 

 with which an ion collides may become very small. 

 The results of the experiments may thus be seen to be 

 in general agreement with the theory, but the variations 

 of a obtained by altering the force and pressure will be 

 examined more fully when the curves connecting a, X, 

 and p are explained. 



