930 Mr. D. C. II. Florance on 



the thickness of radiator from 1*05 cm. to 11 cm. for each 

 position of the electroscope round a quadrant of a circle. 



Eve has shown (Phil. Slag. Dec. 1^01) that for an increase; 

 in thickness of radiator, the emergent secondary 7 radiation 

 increases rapidly till it reaches a maximum, and then it 

 decreases. The curve thus obta : ned can be expressed mathe- 

 matically hv the difference of two exponentials. Mad sen 

 (Phil. Mag. March 1909) has shown the sun.- effect. The 

 "incident" secondary y radiation, i, e. the radiation tnrned 

 hack in it- path, 1 also been shown to be represented by 

 an expression K(l—e~ x ^), where K is a constant, X 2 the 

 coefficient of absorption of this secondary radiation, and d 

 the thickness of the plate. Experimenters in attacking this 

 m have kept their ionization vessel in one position 

 and have made it large to obtain the greatest secondary 

 With 300 mgrs. of RaBr 8 there was sufficient secon- 

 dary radiation to allow measurements to he made with a 

 small electroscope and to be carried out round the arc of a 

 circle. Hence it was Found that instead of a sharp line of 

 demarcation between the emergent and incident secondary 

 7 radiation, the one gradually changed into the other. Curves 

 illustrating this would change in form from that represented 



Considering the complexity of the radiation and the im- 

 perfections of experimental arrangements, it is not to be 



expected that there would be any simple mathematical relation 

 between the quantities measured; and the equations proposed 

 by former experimenters are certainly inadequate. 



Relation between the amount of secondary radiation and position 

 of electroscope for certain thicknesses of radiator (tig. 4). 



In these curves for each thickness of radiator the amount 

 of secondary radiation is plotted radially. By continuing 

 the curves an approximation can be obtained of the amount 

 of the secondary radiation which passes through in the direc- 

 tion of the primary radiation, and also of the amount of 

 "incident" secondary radiation. A similar approximation 

 can be obtained from fig. 3. 



It is important to notice that a considerable portion of the 

 total 7 radiation striking the radiator is converted into 

 secondary. For instance, the leak in the electroscope when 

 there is no radiator is 215 div./min. The leak wdien there 

 is a radiator of iron 2*1 cm. thick is 103 div./min. When 

 the electroscope is in a position just outside the direct radia- 

 tion the leak is 11 div./min. With the present arrangement 

 it is difficult to determine the total quantity of secondary 



