Particles from Thorium Active Deposit. 581 



particles originates in the active deposit. It seems probable, 

 on general grounds, that these particles are a-rays, but this 

 point can only be settled conclusively by some form of 

 magnetic deflexiou experiment, in which the value of e/m, 

 and consequently the mass of the particles, is determined *. 



^> 4. Number of Long-range Particles. 



In 1.916 a determination was made, by two different 

 methods, of the ratio of the number of long-range a-particles 

 relative to the number of ordinary a-particles. The value 

 then obtained, 1 in 10,000, was based on the results of a few 

 observations only, and may consequently be not entirely 

 reliable. Since the publication of Sir Ernest Rutherford's 

 paper on " Collision of a-particles with Light Atoms "f, it 

 became of considerable importance to re-determine this ratio 

 more carefully. 



The first method employed in the earlier determination of 

 the ratio was briefly as follows. The number of long-range 

 particles per minute was measured with the source fixed 

 at a known distance from the zinc-sulphide screen. After 

 the active deposit had decayed in situ for 32 hours, the 

 mica absorbing screen was removed and the number of 

 a-particles from thorium C measured at distances from 6 to 

 7 cm. so as to include only the S'6 cm. a-particles from 

 thorium 0. Reducing both sets of observations to the same 

 distance from the active source and correcting for the decay 

 in 32 hours, a value of the ratio could at once be deduced. 

 In the second method the source and zinc-sulphide screen 

 remained fixed at the same distance apart (about 5 mm.) 

 throughout the measurements, a comparison being made 

 between the number of long-range particles penetrating the 

 mica screen initially and the total number of a-particles 

 (5*0 and S 6 cm.) from thorium six days afterwards (the 

 mica screen being removed). Assuming the decay period 

 of thorium active deposit to be 10'6 hours, the number 

 of ordinary a-particles at the commencement of the ex- 

 periment could easily be obtained and the required ratio 

 evaluated. 



Recent experiments have proved the latter to be the more 

 reliable method, the results now given being based entirely 



* The magnetic deflexion of the long-range particles lias since boon 

 accomplished by Sir Ernest Rutherford, who has proved conclusively 

 that the long-range particles are ir-rays. 



t Phil. Mag. June 1919. 



Phil. Mag. S. G. Vol. 41. No. 214. April 1921. 2 Q 



