﻿24:8 Mr. L. V. King on Absorption 



The results are shown graphically in Plate VII. fig. II. These 

 carves show very distinctly that even in the case of distri- 

 bution of radioactive matter throughout an infinite thickness, 

 practically the whole effect on the gradient is due to a layer 

 very little more than 11 cm. thick. This point is of some 

 importance in an actual measurement of the gradient. Unless 

 the penetrating radiation is measured over a fairly level and 

 homogeneous area, we cannot expect (12) to give an adequate 

 representation of the gradient *. 



§ 4. It is interesting to notice that a considerable number 

 of absorption problems for flat plates involve the exponential 

 integral. Several such problems are considered by Soddyf. 



Fig. 3. 



£ 



The results (due to Sir J Larmor) are given for a point- 

 source of radium in contact with a plate of thickness h. In 

 the notation of the present paper the ratio of the total radia- 

 tion over an angle of 180° on the opposite side of the plate to 

 that which would exist if the plate were removed is given by 



h = e - A + K~h Ei(-Kh)=f{tc7i). 



lo 



If the radiation be confined to a cone of semi-vertical 

 angle 0. we have 



J / = /(*&)— cos fl/(*7i.«pc-fl ) 



V 1— CO* d 



* Cf. Wulf, Phys. Zeit. Sept. 15, 1910, p. 811. 



Hess {luc. cit.) finds from observations taken during- a balloon 

 ascension that there is no appreciable diminution in the intensity of the 

 penetrating radiation with height as far as 1000 metres. From the remarks 

 of the present section it would appear that a comparatively small amount 

 of solids containing- radioactive matter (e. g. sand -ballast &c. taken up 

 in the car) might give rise to enough penetrating- radiation to masK 

 the effect due to height above the earth's surface. 



t Scddy (P. & W. M.) and Russell, " The Question of the Homogeneity 

 of y-ravs/' Phii. Mag. xix. May 1910, p. 725. 



