a. Bays from Radium and Polonium. 



547 



the camera end, where the field gradually fell off, the whole 

 path of the rays was through a uniform magnetic field H of 

 6235 c.g.s. units. 



Fis;. 5. 



vc bumb <- 



Photographic plates were exposed with the field first in 

 one direction and then reversed, the exposure for each side 

 requiring from one to two days. There was no noticeable 

 dispersion of the beam of rays, so that the rays given off by 

 polonium have all the same velocity. A copy of one of the 

 negatives from which measurements were made is shown in 

 figure 6 (PI. VIII.). 



The double deflexions observed on two plates were '71 and 

 *73 cm. respectively, giving as the mean deflexion to either 

 side of the centre '36 cm. The calculated radius of curva- 

 ture R is 53'0 cm. 



Hence the value of — =HR=3'30x 10 5 . 



e m 



If we assume that the value of — is the same for these 



e 



particles as for those from radium, we see that their velocity 

 is greater than that of the average a. particle from radium, 

 but not so fast as that of the fastest. On page 171 of 

 Rutherford's paper in the July number of this Magazine, 

 he calculates that a particles which have a maximum range 

 in air of 4 cm., which is about that of the polonium rays 3 

 will have a velocity of 88 per cent, of that of the a rays 

 from radium C. Taking Rutherford's value of 3"98 X 10* 



as the value of 



for radium C, this would give the corre- 



2P2 



