Produced by Rontgen Bays. 41 



under the influence of an externally impressed field. The fol- 

 lowing method was devised by Langevin for the experimental 

 determination of e. The gas between the two parallel-plate 

 electrodes was ionized by a single flash of very short duration 

 from a Rontgen ray bulb ; the ions are regarded as being 

 formed uniformly in layers parallel to the electrodes although 

 the density of ionization may vary from one layer to another. 

 The charge communicated to the electrodes for electric fields 

 of different intensity is measured by an electrometer ; the 

 electric field between the plates is in general chosen very small 

 so that appreciable recombination may take place during the 

 passage of the ions through the gas. The charge Q brought 

 over by a field X is given by the formula 



X . / ■ , 4ttQ € 



Q = -log(l + 



X 



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 x and & 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 inde- 

 pendent of the pressure below one atmosphere. This dis- 

 crepancy 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 experimental 

 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 dis- 

 tribution. The error becomes greater as the pressure is 

 reduced. 



(3) Variations in the intensity and penetration of the Ront- 

 gen 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 Rontgen ray bulb, allowing the ions thus produced to 



* Loc. cit. 



