produced ly j3 1 J articles from Radioactive Substances. G05 



velocity exceeds only slightly the velocity of hard cathode 

 Fays. 



As to the actual number of /5 particles expelled from each 

 disintegrating atom very little is known. Direct determi- 

 nation of the charge, which would give such information, 

 has so far been made only in the case of the active deposit of 

 radium, and, at the present time, it appears to be a matter 

 of considerable difficulty to carry out similar experiments 

 with other radioactive materials on account of the fact that 

 most of them are obtainable only in small quantities. 



In the present paper we hope to throw some light on these 

 problems from other considerations. If we assume for the 

 present that the ionization produced by a /3 particle does not 

 depend appreciably on its velocity, we can determine the 

 relative number of ft particles emitted from the various 

 substances by measuring the ionization produced by them. 

 Consider a /3-ray product in radioactive equilibrium with an 

 a-ray product. In this case the number of a particles 

 emitted is identical w T ith the number of atoms breaking up 

 of the /3-ray substance. By measuring the ionization due to 

 the /3 rays and dividing it by the number of a. particles emitted 

 per second from the substance in equilibrium, we obtain the 

 ionization due to the /3 particles emitted from one disintegrating 

 atom. These ratios were determined for all substances for 

 which it was possible to carry out such measurements. 



Experimental Details, 



The experimental method of attacking the problem, as 

 indicated above, divides itself naturally into two parts, 

 namely: the determination o£ the number of a. particles 

 emitted per second and the measurement of the ionization 

 current produced by the /3 particles. 



The determination of the number of a particles can be carried 

 out by the scintillation method or by an electrical method ; the 

 latter method, being found more convenient, was generally 

 used. In this method the active plate was placed in the centre 

 of a silvered spherical glass bulb, 15'9 cm. internal diameter, 

 which was exhausted in all the experiments to a pressure of 

 5-46 cm. of mercury, thus allowing only *571 cm. of the 

 range of the « particles to be effective in producing ionization. 

 At this reduced pressure saturation is easily obtained, and 

 the current measured gives readily the number of ions pro- 

 duced for the effective part of the range of the a particle. 

 Using the data* given by one of us for the ionization 



* H. Geiger, Proc. Roy. Soc A. lsxxii. p. 486 (1809). 



