608 Mr. E. T. Beatty on Secondary 



opposite directions in the two chambers, and by varying the 

 cubic capacity of the chamber B a perfect balance could be 

 obtained. The portion of the wire which ran to the electro- 

 scope was surrounded by an earthed tube. 



A comparison can be made between the primary and 

 secondary beams in terms of a ratio which we shall call the 

 penetrative power of the secondary for any particular thickness 

 of absorbing plate. 



This ratio is got as follows : — Work the bulb and observe 

 the ratio of deflexion in the secondary electroscope to that in 

 the primary in a given time. Let this be 5 say. Then place 

 a plate of aluminium, say '04 cm. thick, at P and another at 

 S (fig. 3), and find the ratio of secondary to primary again 

 for any suitable time of discharge. Let this ratio be 6. 

 Then more ionization has taken place in the secondary re- 

 lative to the primary in the second case than in the first, i. e. 

 the secondary is more penetrating than the primary, and the 

 ratio 6/5 = 1*2 gives a measure of the power, relative to the 

 primary, of the secondary to penetrate a plate of Al '04 cm. 

 thick. We shall call this penetrative power P, and the above 

 result would be symbolically expressed P. (M =1 , 2, where 

 the subscript indicates the thickness of the absorbing plates 

 to which the penetrative power refers. 



Now the primary, which is a heterogeneous beam, can be 

 altered in character by cutting off' the more absorbable rays 

 by plates of Al placed at A (fig. 3). As the soft rays were 

 more and more cut off, it was found that the penetrative power 

 as above defined fell steadily. 



Table I. 



. of absorbing plates 

 at A : each 

 •265 cm. thick. 



*■<*■ 







l-ll 



1 



1-09 



2 



•995 



3 



•909 



Percent, of secondary- 

 rajs cut off by Al plate 

 •04 cm. thick. 



52-8 

 39-8 



In the last two experiments the hardness of the rays given 

 out by the bulb increased considerably, and made it impossible 

 to measure with accuracy the percentage cut off. 



From the table it appears that the secondary rays are 

 initially more penetrating than the primary, but as the softer 

 rays are cut off from the main beam, the penetrative power 



