﻿376 Prof. E. Rutherford and Dr. 0. Halm 



on 



The results of the measurements are. given below, using 

 the same notation as before. The values of d l9 d 2 , d z in this 

 case refer to the deflexions observed for the rays after passing 

 through the aluminium screen : — 



^=1-32; ^ = '85; ^ ;= 3/55. 



«3 «3 "2 



Kemembering that the ranges of the three sets of rays are 

 each reduced 2*5 cms. in their passage through the aluminium 

 screen, the emerging pencils of rays from thorium B, 

 thorium C, and radium C have ranges of 2*5 cms., 6*1 cms., 

 and 4*56 cms. respectively. The ratios, calculated as before, 

 are 



I 1 = 1-245; ^ 2 =-89;- ^=1-38. 



The calculated and experimental ratios are not in as good 

 agreement as would be expected from the excellence of the 

 photographs. It is not unlikely that a part of the difference 

 is due to an accidental shift of the photographic plate in 

 arranging the apparatus for the radium experiment. 



Experiment 3. — A layer of mica equal in stopping-power 

 to about 3*5 cms. of air was placed over the active wires. 

 The photographic effect of the a rays from thorium B was in 

 this case too weak for accurate measurement. The photo- 

 graphic intensity of an a particle appears to be approximately 

 proportional to its range in air. The range of the a rays 

 from thorium B in this experiment was only 1*5 cm., while 

 the rays from thorium C had a range 5'1 cm. The photo- 

 graphic effect of the rays from thorium B was consequently 

 less than one-third of the rays from thorium C. The observed 



value of 4- was 1*12 while the ratio calculated from the 

 d s 



ranges is 1*15. 



Experiment 4. — A layer of aluminium equivalent in 



stopping-power to 1*5 cm. of air was placed on top of the 



mica. The interposed screens consequently corresponded to 



5 cms. of air and completely cut off the rays from thorium B. 



The photographs obtained are shown in fig. 3, magnification 



about 1*4. Fig. 3 a shows the bands due to thorium C and 



fig. 3 b the bands due to radium C The much greater 



deflexion of the rays from radium C is obvious from an 



inspection of the photographs. As we should expect, there 



