Field of Radium B on Recoil from Radium A, 883 



evacuated when required. The wire W, of diameter 05 mm., 

 coated with radium A by exposure to the emanation for 

 ten minutes, was placed at the end of the aperture between 

 the two metal plates A, B, 1 cm. long and 0*5 mm. apart. 

 The " recoil-stream" from W passed through this aperture and 

 fell upon the plate (J. The metal tube K served to prevent 

 disturbances by stray electric charges on the glass of the 

 containing vessel. In its path of 7'1 cms. from W to C, the 

 recoil-stream w T as exposed to a uniform magnetic field. 



To carry out an experiment the active wire W was 

 mounted in the position shown in fig. 1, the glass tube was 

 evacuated as quickly as possible, the magnetic field applied 

 and the recoil-stream from Wallowed to pass between the plates 

 A and B and fall upon the metal strip C. The distribution 

 of the activity on the plate G was subsequently measured in 

 exactly the same manner as in the experiments on the electro- 

 static deflexion by means of an a-ray electroscope. To 

 obtain the magnitude of the deflexion suffered by the radium 

 B while passing through the magnetic field, two experiments 

 were performed, one as described and a second one with the 

 field reversed. The distribution of activity over the plate in 

 these two experiments is shown in PL XVIII. fig. 2, curves I. 

 and II. respectively. Now it had been shown by other expe- 

 riments that the strip C and wire W could be removed and 

 replaced very nearly in the same position, so that the distance 

 between the positions of maximum activity in the two expe- 

 riments just described, gives twice the deflexion suffered by the 

 recoil-stream in each experiment. It will be seen from fig. 2 

 that the distance between the two maxima is *645 cm. The 

 paths of the recoil-streams are circles and the positions of 

 maximum activity Q and R on the strip C are due to matter 

 projected from W describing circular paths passing through 

 WPQ and WPP respectively. If PC = ^ and WY = d 2 , 

 then if QR = d and p is the radius of curvature of the path 

 of the rays, we have that 



dp = d 1 (d l + d 2 ). 

 Since d 1 = 6'l cm. and d 2 = l cm., it follows that 

 /3 = 67*2 cm. 



Since for the experiments described the strength of the 

 magnetic field was 10,800 gauss we have, with the ordinary 

 notation, that 



^=Hp = 7-26xl0 5 . 



e r 



