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XXIX. The Velocity of the ol Particles from Radium A. 

 By X. Tunstall and W. Makower *. 



[Plate V. fig. 2.] 



THE velocity of the a particles from radium C has 

 recently been very accurately determined by Ruther- 

 ford and Robinson f. The value given is 1*922 x 10 9 cm. 

 per second, and from this number the velocities of the 

 a particles from other radioactive substances are calculated 

 by Greiger's formula 



v> = m, 



which gives the relation between the velocity v of an 

 « particle and its range R in air. The quantity k is a 

 constant. 



A direct comparison of the velocity of the a particles from 

 radium A with that of the particles from radium C, does not 

 appear as yet to have been made ; and as the apparatus 

 used to measure the magnetic deflexion of the recoil stream 

 from radium A J was also suitable for this purpose, ex- 

 periments were made to determine the ratio of the velocities 

 of the a particles from radium A and radium C as accurately 

 as possible. 



The apparatus used was the same as that used by 

 Walmsley and Makower §. The wire (fig. 1, p. 253) was 

 made active by exposure for eight minutes to radium emana- 

 tion and quickly mounted in the apparatus, which was then 

 evacuated as rapidly as possible. The a rays passing 

 through the slit fell on the photographic plate, which 

 was subsequently developed. A field of about 14,000 gauss 

 was applied between the pole-pieces of the magnet. The 

 a rays from the radium A on the wire thus fell on the 

 photographic plate, producing an image of the slit. After 

 four minutes the field was reversed for fifteen minutes, 

 and a second image due to the a rays from radium A was 

 obtained. During the interval which had elapsed since the 

 exposure of the wire to the emanation, sufficient radium C 

 had grown on the wire to produce simultaneously on the 

 photographic plate a second and slightly less deflected 

 image. By this time all the radium A on the wire had 

 decayed, and the field was again reversed for ten minutes 

 so as to obtain an image from the a rays from radium C on 



* Communicated by the Authors. 



t Rutherford and Robinson, Phil. Mag. Oct. 1914. p. <322. 



t Walnish 



§ Loc. cit. 



S2 



