loNIZATION BY POSITIVE IONS 47 



X be acting in each case the energy acquired between 

 the collisions will also be the same. The relative 

 ionizing powers may therefore be found by comparing 

 the value of a in a gas at a certain pressure with the 

 value of ft in the gas at a quarter of the pressure when a 

 constant force X is acting. Take as an example the 

 case of hydrogen. With the force X=50 volts per 

 centimetre and pressure 1 millimetre a=-35. The 



Q ~\r 



curves giving ft show that ='090 when =200, so 



that /3 = *022 when X=50 and _p='25. Hence when 

 positive and negative ions collide an equal number of 

 times with molecules and acquire equal kinetic energies 

 along their free paths the negative ion produces sixteen 

 times as many ions as the positive ion. The ratio of the 

 activities of the positive and negative ions as estimated 

 in this way varies with the force X. Under a force of 

 75 volts per centimetre a=*9 when p=l, and /3='065 

 when _p=*25, so that the effect of the negative ion is 

 fourteen times that of the positive ion. 



The rate of increase of the ionizing power of positive 

 ions is therefore greater than that of negative ions, and 

 for very large forces ftjp would theoretically be four times 

 as great as the maximum value of alp. But it is 

 impossible to test the accuracy of the theory on this 

 point, since the smaller values of ft do not supply 

 sufficient data for the purpose, and the larger values 

 cannot be obtained by the experimental method that has 

 been used, as sparking takes place before ftjp rises above 

 a small fraction. The above figures nevertheless show 

 that for small values of the kinetic energy the negative 

 ions are much more efficient than positive ions in pro- 

 ducing new ions by collision. 



