531 On the Absorption Coefficient of Iron for y Rays. 



An experiment was made heating the furnace alone, and 

 the ionization was the same at high and low temperatures. 



It will be seen that the ionization in the electroscope is 

 slightly greater at high than at low temperatures. The 

 mean of a number of experiments showed the rate of increase 

 of ionization to be *0020 per cent, per degree centigrade. 

 This can be shown to be due to the change in density of the 

 iron as follows. 



Let I be the intensity of the rays falling on the block. 

 The intensity, I, of the emergent rays is given by l — l e~ kl , 

 where / is the thickness of the block and X the coefficient- of 

 absorption. 



Now X is proportional to the density of the block, and 

 therefore at any temperature 6 degrees higher is given by 



.. — Fr—Ti where a is the coefficient of linear expansion of iron. 

 1 + oau » 



Similarly the thickness of the block at this temperature is 

 (1 -\-a6)L We therefore have for small values of ud 



I = I o >-AZ(l-2«0) 



o" 



The percentage rise in ionization per degree centigrade is 

 therefore 200a\l. 



In an actual experiment I was found to be 8*11 div. per 

 min. and I 3" 55 div. per rain. 



.-. \Z'=logI -logI = -825. 



Taking a as -0000117 we obtain an increase in ionization 

 of -00193 per cent, per degree, which is in good agreement 

 with the value *0020 obtained experimentally. 



Thus the temperature of the iron within the limits examined 

 affects the absorption coefficient by an amount to be expected 

 from considerations of the change of density of the absorbing 

 substnnce. That is, the absorption of 7 rays by iron depends 

 only on the actual number of atoms traversed by the rays 

 without regard to the temperature of the body which they 

 compose. 



These experiments were carried out in the Physical Labo- 

 ratories of the University of Manchester, and I wish to 

 express my best thanks to Prof. Rutherford for the kind 

 interest he took in them. 



Gonville and Cains College Cambridge, 

 Februarv 1911 



