606 Drs. Geiger and Kovarik on Relative JSumber of Ions 



produced by a. particles of different ranges, the number of: 

 a. particles emitted per second from the plate was calculated. 

 The numbers thus obtained were checked in several expe- 

 riments by counting the scintillations produced on a zinc 

 sulphide screen, and a good agreement was obtained. 



An accurate determination of the /3-ray ionization is beset 

 with numerous difficulties. It is obviously necessary to cover 

 the active material to absorb the a radiation. Since in the 

 thin layer of aluminium used for this purpose an appreciable 

 number of /3 particles is stopped, an extrapolation must 

 necessarily be made to obtain the correct value for no 

 absorption. This correction was made by determining in 

 each case the absorption curve of the /3 rays in aluminium. 

 The method adopted for obtaining the absorption curve was 

 similar to that described previously by one of us *. This has 

 the advantage of avoiding corrections for the reflexion of the 

 /3 particles by the absorbing foils. In addition the air inside 

 the electroscope also absorbs some /3 particles. This was 

 taken into account by further extrapolation for one-half of 

 the mean path of the /3 particles in the electroscope. 



The electroscope used was a copper hemisphere 20 cm. 

 in diameter. The active material was placed in the centre 

 so that the average path was equal to the radius. Since 

 the /3 particles are diffusively reflected from the interior 

 surface of the electroscope, it was necessary to make a cor- 

 rection for this reflexion. This was done by constructing 

 a similar electroscope of the same size made of thin silvered 

 paper in which the reflexion was very small. The amount of 

 this correction was determined by finding the increased 

 reflexions due to increased thicknesses of such paper. The 

 correction varied somewhat for the different types of harder 

 /3 radiations ; but owing to the difficulty of obtaining an 

 accurate value for /3 rays of different velocities, the mean of 

 all the values obtained by u-ing different types of rays was 

 employed in all cases. This correction was 27 per cent, of 

 the ionization produced in the metal hemisphere. 



In order to check the accuracy of the numerous corrections 

 which were necessary, similar experiments were made with 

 electroscopes of widely different dimensions. In addition to 

 the hemisphere two electroscopes cubical in shape whose 

 lengths of edge were 12*7 and 3**4 cm. respectively were 

 used. The active material was placed on the top of the 

 electroscopes. The average path of the /3 particles in each 



* A. F. Kovarik, Phil. Mag. xx. p. 849 (19-10). 



