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LXXVI. The Absorption of Homogeneous ft Rays. 

 By R. W. Varder, M.A. (Cape) *. 

 ^fl^HE experiments described in this paper were made to 

 JL investigate as accurately as possible the form of the 

 ionization absorption curve when very homogeneous ft rays 

 pass through a standard substance like aluminium, and also 

 to test whether there is any simple relation connecting the 

 absorption with either the velocity or energy of the ft 

 particle. 



The apparatus used is shown in fig. 1. 



Fig. 1. 



TO ELECTROSCOPE 



B is a brass box about 3 cm. wide and 10 cm. high. It 

 was placed between the poles (17 X 10 cm.) of a large electro- 

 magnet. The position of the pole-pieces is denoted by the 

 broken line MM. Rays from a thin-walled tube A filled with 

 radium emanation describe circles in the magnetic field ; a 

 pencil of homogeneous ft rays passing through the slit S 

 could be concentrated at C. The possibility of obtaining very 

 homogeneous ft rays by this method has been shown by 

 Rutherford and Robinson f, who employed it to determine 

 the ft ray spectrum of radium B and C. In the present 

 experiments the relative positions of the source A, the slit S, 

 and the opening C were arranged to obtain a concentrated 

 and nearly homogeneous beam at C. The rays there passed 

 through a rectangular slit 1 cm. x 2 mm. in a brass plate 

 and through the mica window 1). A ground-glass plate V 

 placed on the top of the box made it airtight. In order to 

 prevent scattering and diminution in intensity, the air- 

 pressure in B was reduced to about 1 cm. of mercury. The 

 window D bad a stopping power corresponding to about 

 2 cm. of air at N.T.P. In this way a strong pencil of ft rays 



* Communicated by Sir E. Rutherford, F.R.S, 

 t Rutherford and Robinson, Phil, Mag. Oct L913, 



