eve] primary and secondary gamma rays 47 



To calculate the effect of reversing plates A and B we have to 

 look at Table III. or at curve II., fig. 3, and thence obtain the fact 

 that the reversal from Al, Pb to Pb, Al decreases the ionization 

 from 100 to 98. This explains the change from I. to II., and also 

 from III. to IV., just above. 



To calculate the effect of reversing C and D we see in Table I., 

 or from curve I., figure 2, that lead at C gives 100 whereas aluminium 

 at C gives 84. 



Thus II. and IV. are 98 per cent of 1. and III. respectively, 

 but III. and IV. are 84 per cent of I. and II. respectively. Hence, 

 of course, the product of the extremes I. and IV. must equal the 

 product of the means II. and III. The calculated and observed 

 values for Al, Pb are respectively: — 



I. II. III. IV. 



100 98 84 82 



100 98.5 83.5 81.1 



Or again taking aluminium and copper plates: — 

 observed 100 103.5 94.3 96.8 



calculated 100 103.4 93.4 96.6 



In the following Table IV. are given the results for many sets 

 of four plates, and successive columns show, — reference number, 

 nature of screen, thickness, and distance from the radium to the 

 near face of the electroscope. In the fifth column the substances 

 used as plates are set forth, and then follow, — the results for the 

 arrangements I., II., III., IV. as explained above, the products of 

 the extremes, and of the means, also their ratio, which has an average 

 value 0.993. 



