748 Prof. C. G. Barkla and Mr. G. Shearer on the 



Increasing the pressure, the curve became straight as at P 

 when the corpuscular radiation from the ends was totally 

 absorbed. Thus at a pressure p the greatest distance 

 traversed by electrons from the ends was just the length of 

 the chamber, 3*0 centimetres. 



As the front face was of paper, the gain or loss of electrons 

 from this was negligible ; and the surface of the electrode 

 exposed was so minute that its effect also was negligible ; 

 so that the curvature of the graph was due entirely to the 

 net gain of corpuscles from the screen. 



The curves obtained with screens of calcium, zinc, and 

 tin when exposed to Ag and Sn X-radiations (series K) 

 were carefully compared. Although the exact shape of the 

 curve depended upon the substance of the screen, the 

 pressure at which the curve became straight was identical 

 for all screens exposed to a particular radiation. In some 

 experiments it was impossible to determine this point with a 

 possible error less than about 10 per cent., but in others (in 

 which the observations were more consistent) it appeared 

 that a difference of 5 per cent., if such existed, would be 

 detected. 



The results of these experiments showed that the electrons 

 which under exposure to Ag X-rays (series K) were emitted 

 by Ca, Zn, and Sn in a direction straight across the chamber, 

 were just absorbed at a distance of 3*0 cm., when the 

 pressure was 17*5 cm. of mercury. From Whiddington's 

 law and the observed agreement, it was seen that the speeds 

 of the fastest electrons from Ca, from Zn, and from Sn when 

 exposed to Ag X-radiation (series K) did not differ by as 

 much as 2 per cent.* A similar agreement was observed 

 under exposure to Sn X-rays (series K) 5 the pressure p being 

 25 cm. of mercury. (There is very strong experimental 

 evidence in support of the assumption generally made that 

 when the X-radiation is homogeneous the maximum velocity 

 of emergence is the actual velocity of every directly expelled 

 electron as it just leaves the atom.) 



We conclude, therefore, that the variation in the maximum 

 velocity of ejection observed in the case of emission under 

 exposure to ultra-violet light is not characteristic of the 



* The absolute value of the maximum velocity obtained from Whid- 

 dington's formula v 4 = 2xl0 40 Xd, where v is the initial velocity of an 

 electron of path d in atmospheric air (O.G.S. units) is 108x10 s cm. 

 per sec. for the electrons ejected by Ag X-rays (series K), and 118"5xlO s 

 cm. per sec. for those ejected by Sn X-rays. These values agree well 

 with Whiddington's general conclusion that the maximum velocity is 

 10 s timen the atomic weight of the substance emitting the characteristic 

 X-radiation. 



