630 



Mr. W. F. Rawlinson on the Decrease in 



a platinum wire which had been exposed to radium emanation 

 was used as a source when working with the low velocity 

 lines o£ radium B. The wire was fixed in a shallow groove 

 cut in a piece of brass, and the stopping-screen was placed on 

 top of this in order to be as close as possible to the wire,, 

 since this reduces scattering by the material. For the high 

 velocity lines of radium G the a-ray tube was used as a 

 source, since in this case the stopping by the thin glass walls- 

 is negligible. 



The experiment resolved itself into measuring the distance 

 between the two lines. For let p equal the radius of curva- 

 ture of the path of an unstopped /3-ray with any given initial 

 velocity, and pi equal radius of curvature of the path of a 

 /3-ray starting with the same initial velocity, but which h as- 

 passed through the screen. 



Fur. 2. 



Let a = depth of source below slit (see fig. 2). 



d= distance of the line from the centre of the slit. 

 d x — distance of the displaced line from the slit. 



Then 



4rp 2 = a 2 + d 2 

 ±p? = a 2 + d?- 

 Hp 2 - Pl 2 ) = d^d l 2 

 ±(p + p l )(p-pi) = (d + d 1 )(d-d 1 ) 

 .'. §pxSp = 2dx Bd 

 '.' p and p t are nearly equal, 



.'. Bp=f8d. 

 ±p 



S(R P ) 



dR 

 Ap 



Bd. 



Since Bohr's calculations refer to the distance between 



