Ions produced by ft and y Radiations from Radium. 333 



B and radium C " *, and this enables us to arrive at a more 

 satisfactory estimate of the total 7 ray ionization. 



For reasons which will be explained later, the value of nr 2 

 at the distance of 3 metres (Table I.) has been made the basis 

 of our calculations. Assuming the effect of 3 metres of air 

 to be equivalent to that of *15 cm. of aluminium, this corre- 

 sponds to the effect after absorption by 1/1 cm. of aluminium. 

 From the curves of Rutherford and Richardson it appears 

 that of the ionization current observed under these condi- 

 tions 81 per cent, is due to radium C, and 19 per cent, to 

 radium B. 



Taking the total ionization current at this point to be 100, 

 the effects corresponding to radium C and radium B with no 

 absorbing material are found from the curve to be 92*8 and 

 30*5 respectively. (This does not include the very soft 

 7 radiation of radium B, with absorption coefficient 40 cm. -1 

 in aluminium f.) 



Taking the value for nr 2 at 3 metres — 5*28 x 10 9 — from 

 Table I., the value of the constant K for the 7 rays of 

 radium C comes out to '928 x 5*28 x 10 9 



= 4-89 xlO 9 . 



The corresponding value for radium B is *305 x 528 x 10 9 



= 1-61 xlO 9 . 



Before basing any calculations on these figures it will be 

 well to discuss the serious discrepancy between our results 

 and those of Eve J, who in three distinct series of experi- 

 ments found the values 3'8xlO n , 3'74xl0 9 , and 3-81 xlO 9 

 for K. These are much lower than our value, which is 

 moreover taken from the lowest result in our table, and 

 if the 7 rays of radium B were not entirely suppressed in 

 the experiments of Eve, the discrepancy is still more serious : 

 it is far too large to have arisen in the measurement either 

 of the quantity of radium or of the ionization currents, and 

 the same radium standard (the Rutherford-Boltwood) was 

 used in both cases. The chief point of difference lies in the 

 methods used to stop the primary ft radiation. In our 

 experiments the effect of the secondary ft radiation excited 

 in the aluminium screen was superimposed on that due to 

 the rays excited in the air alone. By placing the source 

 and screen in a magnetic field, this effect was measured, and 



* Rutherford and Richardson, Phil. Mag. xxv. p. 722 (1913). 

 t Rutherford and Richardson, loc. cit. 

 % Eve, loc. cit. 



