Properties of the a. Rays from Radiui 



167 



deflexion from the normal of the pencil of rays by the 

 magnetic field. The strength of the magnetic field was about 

 9470 c.G.s. units, and this produced a well-marked separation 

 of the bands. For example, when the photographic plate 

 was 4 cms. from the slit, the distance between the bands on 

 the plate was 4*7 mms. 



The ft and y rays from the active wire were found to pro- 

 duce only a small photographic effect compared with the a. rays 

 themselves. The width of the band on the plate was found 

 to be the same whether the magnetic field was on or not. 

 This shows that the rays were all bent to the same extent, 

 that is, that the a particles were all projected at the same 

 speed. This result has been observed in a number of ex- 

 periments, and confirms the conclusion arrived at by Bragg 

 in a different way, viz., that all the a particles from one product 

 are projected with the same velocity. We may conclude 

 from this that each a particle, at the moment before its ex- 

 pulsion from the atom, is moving at a definite velocity within 

 the atom which is the same for all the atoms of that particular 

 substance. 



The radius of curvature of the path of the rays is readily 

 calculated. 



Let 2d = distance between centre of bands. 

 d 1 = „ of plate above slit. 



d 2 = 



of slit above the beginning of the magnetic 

 field. 

 p = radius of curvature of path of the rays. 



Then 2pd=d 1 (d ] + d q )* 



The following table shows the results obtained for values 

 of d 1 of 2, 3, and 4 cms.; d 2 =l'0 cm. in all cases: — 



Distance d l of 

 plate from slit. 



Value of 

 2d. 



Radius of curvature p 

 of path of rays. 



2 

 3 

 4 



1*465 mms. 



2-78 „ 

 4-79 „ 



41-0 cms. 

 43-2 „ 

 41-7 „ 



Mean value... 42*0 cms. 



The values of p are thus in as good agreement as could be 

 expected, since the magnet was excited by a 110 volt power 

 circuit, whose B.M.F. occasionally fluctuated. These results 

 show that over the whole range examined the path of the 

 rays is a circle of mean radius of curvature of 42'0 cms, 



