68 Dr. S. J. Plimpton on the Recombination of 



charge Q brought over by a field X is given by the 

 formula 



X , /. . 47rQ„e N 



^rX^TF)' 



where Q is the charge corresponding to a field of sufficient 

 intensity to prevent appreciable recombination. 



Applying this method, and making use of the previously 

 determined values of k ± and k 2 , Langevin found that the 

 values of a, showed a considerable falling off when the air- 

 pressure was reduced below one atmosphere ; whereas the 

 experimental results obtained by McClung *, who used 

 the method described above, had assigned a value to a 

 which was practically independent of the pressure below 

 one atmosphere. This discrepancy was almost certainly 

 due to the abnormally large corpuscular radiation which 

 proceeded from the electrodes in McClung's experiment 

 when the Rontgen rays were acting. 



It is convenient to indicate here briefly three main sources 

 of error which are liable to occur in the course of experi- 

 mental work on this subject : 



(1) Diffusion of the ions. 



(2) Presence of corpuscular radiation proceeding from 



the electrodes. 



This gives an abnormally high density of 

 ionization near the plates, making the value of a. 

 greater than for a uniform distribution. The 

 error becomes greater as the pressure is reduced. 



(3) Variations in the intensity and penetration of the 



Pontgen rays due to continued working of the 

 bulb. 



The method adopted in the present series of experiments 

 reduces these sources of error to a minimum. It consists 

 essentially in ionizing the gas between two parallel plate 

 electrodes as uniformly as possible by means of a single 

 flash from a Pontgen-ray bulb, allowing the ions thus 

 produced to recombine in the absence of any external field 

 for small intervals of time which can be regulated and 

 determined mechanically, and by the sudden application of a 

 strong electric field driving over to the electrodes those ions 

 which remain in the gas. In this way we obtain a series of 

 values of n corresponding to different values of t, and values 



* Loc. cit. 



