loNIZATION BY NEGATIVE IONS 



27 



TABLE VHI. 

 HELIUM. N= 2 -4; V=14'5. 



The above tables include the results of the most 

 accurate experiments, and are probably correct to within 

 2 or 3 per cent. The values of a/p for the large values of 

 X/p given in some of the papers are not very reliable, so 

 they have not been included. The difficulty arises in 

 these cases from the action of the positive ions. When 

 the positive ions are not producing any appreciable effect 

 the value of a can always be determined accurately by 

 taking the ratio of the currents at two different distances 

 between the plates, even when the free path of the ion 

 is a considerable fraction of the smaller distance, as it is 

 only the change of distance which is involved in the 

 calculation. With large forces and small pressures, 

 however, it is necessary to take into consideration the 

 action of the positive ions. In this case the expression 

 for the ratios of the currents at different distances 

 involves the absolute distance, and for accurate deter- 

 minations it is desirable to use distances between the 

 plates which would be large compared with the free 

 paths. But there is a limit to the extent to which such 

 experiments can be carried, for with large forces and 

 small pressures sparking takes place when the distance 



