﻿254 Mr. Walinsley and Dr. Makower on Magnetic 



field will be bent into circles, and will strike the screen in lines 

 parallel to the source and slit but displaced from the point P. 

 I£ the distance OS = Zi and the distance SP = Z 2 , then if the 

 displacements d a and d r suffered by the a particles and recoil 

 stream respectively are measured, the radii of curvature are 

 given by the equations 



P .=M l > +d °}{^ +d "}\ (2) 



and *«i}v+*'}{<W+**}J J ' • 



whence o a d r c -, d a " — d r da," — d r ) 2 . . •. 



pa = r 1+ _a + . ^ 2 approximately. 



.... (3) 



If, therefore, the quantities d a and d r are measured, the 

 radius of curvature of the recoil stream in the magnetic 

 held can be compared with that of the a. rays and the value 



£ \ — ~ m ) f or ^ ne recoil stream found without the neces- 



e 

 sity for an absolute determination of the value of the magnetic 

 field or for measuring the quantities Z x and l 2 with great 

 accuracy, since these quantities occur only in small correcting 

 terms. 



The method depends upon the possibility of detecting the 

 positions at which the a rays and recoil stream strike the 

 screen P. This can be done easily in the case of a particles 

 by using a photographic plate as the screen. Various plates 

 were tried to see whether the recoil stream gives a develop- 

 able image, but the tests, though not exhaustive, failed. It 

 was found, however, that under suitable conditions, if the 

 radium B was allowed to decay on the photographic plate 

 in situ, a photographic image was obtained by the subsequent 

 emission of a rays from the radium formed from it. 



The apparatus consisted of a stout brass box, the internal 

 dimensions of which were 18 cm. X 3" 7 cm. X 09 cm. (fig. 2). 

 The box contained the slit S and the photographic plate P. 

 It was closed at one end by means of a piece of plate glass A 

 o-round on to a broad flange and at the other by a closed 

 ground joint B. To this was attached on the end of a brass 

 rod C a plate which carried the active wire. The apparatus 

 was evacuated through the vertical tube J), which was pro- 

 vided with another joint by means of which it could be 

 detached from the rest of the apparatus when necessary. The 

 slit S was horizontal and was 0*1 mm. wide. It was mounted 



