a Particles from Thorium Active Deposit. 



703 



comparison between the velocities o£ the a-particles emitted 

 from radium A and radium C, by measuring carefully the 

 deflexion of the path of these a-par tides in strong magnetic 

 fields. Using the value given by Rutherford and Robinson 

 for the velocity of a-particles from radium G, the value 

 obtained for the velocity of a-rays from radium A is in close 

 agreement with that calculated by Geigers formula. 



The work described in the present paper is incidental to a 

 research, in conjunction with Dr. W. Makower, on the 

 photographic effect and magnetic deflexion of recoil atoms 

 from radium C and thorium C. In these experiments the 

 magnetic fields used to deflect the recoil atoms are sufficiently 

 strong to produce a fairly wide separation of the two groups 

 of a-particles emitted from thorium active deposit. Accurate 

 measurements of the deflexions produced by these magnetic 

 fields lead to a very consistent value for the ratio of the 

 velocities of the a-particles from thorium C 2 (a-particles of 

 8*6 cm. range) and thorium C, (a-particles of 4*8 cm. range). 



By using a combined source of thorium and radium active 

 deposits a direct comparison of the velocities of the a-par- 

 ticles from thorium Oj and thorium 2 has been made with 

 the standard velocity of a-particles emitted from radium C. 



1. Determination of the ratio of the velocities of 

 a-particles from thorium active deposit. 



The apparatus used was similar in principle to that 

 employed in previous determinations of this character. A 



Fur. 1. 



AJ 



n^f 



Vr 



pp. Section of pole-face. dd. 



a. Active wire (0-loG mm. diam.). c, 



b. Slit (O067 mm. wide). 



c. Photographic plate. 



Slotted bar (graduated in mm.). 



Light-tight tube leading to 

 vacuum pump and pres- 

 sure gauge. 



