﻿424 Tracks of the a. Particles in Sensitive Films. 



is quite possible that the developed grains follow one another 

 with too wide intervals to present themselves as a track o£ a 

 j3 particle, much more so when the liability of scattering of 

 the rays is considered. It will be of interest to recall that, 

 in the well-known experiment of C. T. R. Wilson, ;i 

 considerable difference was observed between the a and 

 (3 particles in forming a track in air. We are inclined to 

 believe that the grains spreading out over a wide region 

 in a far less definite manner are to be attributed to the 

 /3 particles. 



We shall next describe some experiments made in magnetic 

 fields. A photographic plate was brought into contact with 

 an active needle in the way already stated and quickly intro- 

 duced between the poles of a powerful electromagnet, the 

 plate being held perpendicular to the lines of magnetic force. 

 When the plate seemed to have been exposed to a sufficient 

 number of a particles, it was withdrawn and developed 

 immediately. Working, however, in a field up to nearly 

 ten thousand gauss, no indication of curving of the track 

 was observed. Taking the velocity of the a particle as 

 2 X 10 9 cm. per sec, it can be shown that the particle 

 should, in a field of 10,000 gauss, describe a path for which 

 the radius of curvature is as great as 40 centimetres. Such 

 a slight curvature would not easily be recognized, as the 

 length of the path under examination is so minute, that it 

 is only about •054 mm. 



Evidences have already been given by the previously cited 

 investigators that many of the a particles suffer sudden 

 deflexions on the passage through the emulsion film. On 

 the microphotographs taken by Walmsley and Makower, 

 this effect is seen exceedingly well. This phenomenon can 

 be observed on our photographs too. Fig. 8 (PL VII.) 

 is the microphotograph of a plate in which the a particles 

 passed from left to right, the magnification being in this 

 case 1,210 diameters. It can be seen that, while one of the 

 a particles passed straight on, another suffered a sudden 

 deflexion of about 15° after traversing some distance nearly 

 parallel to the former. It may be remarked, that we have 

 examined a great number of sets of the radial tracks of the 

 a particles, but so far we have not been able to find any 

 which can he said with certainty to have suffered the deflexion 

 of an angle so large as 90°. The number of the tracks 

 showing such a large deflexion seems to be one in several 

 thousands at most. 



In the halo with a thrust-mark inside, such as that in 

 fig. 5, many of the radial tracks are seen to be curved in 



