Ions produced by /3 and y Radiations from Radium, 329 



radium emanation, which Mr. Chadwick very kindly com- 

 pared with the laboratory radium standard by means of the 

 7-ray balance f. The emanation was contained in a thin 

 glass tube, whose walls only stopped the a. particles by an 

 amount equivalent to 2 cm. of air> so that the absorption of 

 /3 radiation by the glass was quite slight. Measurements 

 were made of the ionization produced by the j3 and 7 radia- 

 tions together at distances from 10 cm. to 9 metres, and in 

 many cases were repeated with 1 cm. of aluminium sur- 

 rounding the source in order to find the effect of the 7 

 radiation alone. At short distances the rapid variation of 

 n necessitated the use of a very small ionization chamber, 

 while at long distances a large chamber was needed to obtain 

 a measurable ionization current. Three chambers, A, B, and 

 C, were therefore used in turn. 



Chamber. 



Kange of use. 



Approximate 

 dimensions. 



Walls Frame- 

 made of work of 



1 



A 



10 cm. to 50 cm. 

 30 cm. to 3 m. 

 1 m. onwards. 



1x2-8x2-4 cm. 

 7-2x7-1 x7'15 cm. 

 20-6 X 20-6x20*5 om. 



Al leaf. Fine wires. 



B 



Al foil 



Fine wires. 







•002 mm. | 

 Tissue- Steel 





paper lined 

 Al leaf. 



knitting- 

 needles. 



The wire frameworks of A and B were stretched inside 

 cages of steel knitting-needles, and each chamber was fixed 

 on light supports at about 2 metres from the laboratory 

 floor. Eve f has shown that under similar conditions the 

 presence of the ionization chamber has no appreciable effect 

 on the radiation, so that a true measure of n is obtained 

 by dividing the ionization current by the volume of the 

 ■chamber. Since the sides of A were somewhat irregular, 

 its volume could not be measured accurately, and it was 

 therefore calculated from that of B by comparing their 

 ionization currents when at the same distance from the 

 source. Each chamber was furnished with a very light 

 central electrode connected with a Dolezalek electrometer. 

 The walls of the chamber were kept at a potential sufficient 

 to ensure saturation, and the ionization current measured 

 with a standardized condenser by Townsend's balance 

 method %. The connecting wire was carefully f1 ^ed, for 



* Rutherford and Chadwick, Proc. Phys. Soc, London, xxiv. p. 141 

 (1912). 



t Eve, loc. cit. 



% Townsend, Phil. Mag. vi. p. 603 (1903). 



