the Absorption of Rontgen Rays. 747 



Such a transformation occurs when a radiation like that 

 haracteristic of Se passes through Zn, Cu, Ni, Fe, &c, or 



the radiation characteristic of Cu is transmitted through Fe. 



<- 



Absorption Experiments. 



In the experiments which were made to determine the true 

 absorption of homogeneous beams, the radiator R, used to 

 produce the beam, was placed at a distance of about 7 centi- 

 metres from a rectangular aperture A, (3 cm. x 2 cm.) in a 

 lead screen. A second lead screen with a similar rectangular 

 aperture A 2 was placed at a distance of 7 centimetres from 

 the other, and behind the second aperture was placed the thin 

 paper and aluminium face of the electroscope used to measure 

 the intensity of the beams. 



The absorbing plates were placed across the aperture A 1 . 



As the radiation from R necessarily produced a diverging 

 beam, only a portion of the energy parsing through Aj passed 

 through the aperture A 2 , consequently the secondary radiation 

 from the plate produced greater proportional effects than that 

 calculated for a parallel beam. The ratio of the ionizations 

 due to the secondary and primary beams had to be multiplied 

 by a factor which expresses the ratio of the energy passing 

 through aperture A x to that passing through aperture A 2 

 when no absorbing plate intervened. This was found to be 

 about 4*9. 



Thus 



[I 



Secondary Ionization . , co k i*V+ — K— Ml 



rrimary Ionization 47r \ 2 — Xj z x L J 



= 4-9 X -01 x &*- — — - — - — - approximately. 



%i X 2 — \ x II 



Now, h - may be got by direct experiment, for — ? is 



merely the fraction of the energy of: primary radiation 

 absorbed which is transformed into or which appears as 

 secondary radiation — as measured by the ionizations produced 

 j n a thin film of air. 



I £ . 2 j has been determined by one of us by direct expe- 

 riment'*^ was determined by using the absorbing substance 

 as a radiator also, and so observing the absorption of the 

 radiation characteristic of the substance by the same sub- 

 stance — this, of course, required no correction for secondary 



* C. A. Sadler : "Transformations of Rontgen Rays," Phvs. Soc. Lond. 

 April 23, 1909. 



