76 Proceedings of the Royal Irish Academy. 



between tlie plates. An ion produced at the edge of the lower plate will 

 therefore be carried over to the further edge of the upper plate by the 

 voltage difference V if 



V 



u — . L 



a 



j\^y, 



L being the length of the plates. But h vdy is the volume of air passing 



Jo 



between the plates per second, & being the breadth of the space. Calling the 

 volume of air per second §, we have 



Lo 



It is here assumed that the air-velocity is constant over any plane parallel 

 to the plates. This is not true, but the error will not be very serious in the 

 experiments described where i was more than three times a. 



From the curves obtained it was concluded that the bulk of the ions 

 consisted of four definite classes, the mobility values ranging from about 

 2 to 1'35. In addition it was found that ions of mobility up to 12 were 

 present in small quantities. 



Blackwood' has endeavoured to repeat these observations, using an 

 apparatus apparently identical with that used by the author. He shows 

 that the greater part of the current in his experiments is due to ions 

 produced throughout the general volume of the apparatus ; that therefore 

 the ions from the restricted source play but a small part. He states that if 

 the general ionization is allowed for, a curve is obtained giving a normal 

 mobility value for the ions. That the general ionization plays a considerable 

 part in the carrying of the current observed (a part the exact magnitude of 

 which will depend on the source of ionization and the nature of the shielding) 

 was not in need of any experimental demonstration. It was clearly realized 

 and is referred to in the author's original paper as follows: — "We have assumed 

 that all the ionization is produced in a certain restricted region ; but the 

 more penetrating radiations will cause a weak general ionization throughout 

 the whole air-space. The current due to this ionization will increase 

 smoothly as the voltage increases, so that our experimental curve will really 

 be due to the superposition of the step-like curve on this smooth curve." 

 It is clear, then, that it was the step-like singularities in the curve which 

 were sought for, and that the general contour of the curve was not a matter of 

 any special interest. When these steps were found they were naturally 

 attributed to the ions from the restricted source ; the current- voltage curve 

 was plotted accurately in the neighbourhood of the singularities and the 



' Ijoc. cit, 



