172 Mr Kleeman, Tlie nature of the ionisation 



The end d of the chamber A was closed with a thin sheet of zinc. 

 £ is a lead block 3 cm. thick through which a circular hole e was 

 drilled about 1 cm. in diameter. The axis of this hole and the 

 glass tube C containing 80 mgrm. of radium (which was surrounded 

 by sheet lead 2 mm. thick), were placed co-axially with the tube 

 ah and the ionisation chamber A. The ionisation in the chamber 

 was produced principally in the cone /' by the 7 rays from the 

 radium and the secondary /3 rays from the plate c, the secondary 

 /3 rays being initially projected in the direction of propagation 

 of the 7 rays*. The secondary /3 rays produced in a plate are to 

 some extent scattered by the plate, but the larger part of the 

 radiation proceeds approximately in the direction of propagation 

 of the rays. Thus the writer has shown in the paper above 

 mentioned that the pencil of secondary /3 rays emerging from 

 a plate placed at one end of an aperture in a thick lead block 

 while the radium is placed at the other end, is almost exactly 

 of the same form as that obtained by shooting the /3 rays from 

 the radium through the aperture. The larger part of the radiation 

 from the plate c therefore entered the chamber. 



The measurements were carried out by first placing a lead 

 plug into the hole e of the lead block B and measuring the leak 

 in the chamber. This gives the leak in the chamber due to 

 imperfect screening of the lead block B. The leak was then 

 measured with the plug removed. The difference between this 

 leak and the former gave the ionisation in the cone / due directly 

 to the 7 rays and the secondary rays from the plate c. A magnetic 

 field of sufficient strength to bend the ^ rays from c so that they 

 did not enter the chamber A, was then applied, and the leak again 

 measured. The difference between this leak and the first gave 

 the leak in the cone approximately due to the direct action of the 

 7 rays. 



Whether the strength of the magnetic field was sufficient 

 to bend the ^ rays from the lead plate c so that they did not 

 enter the chamber A was tested as follows. The plate c and the 

 lead sheeting surrounding the radium was removed so that a pencil 

 of fi rays from the radium now penetrated into the chamber. 

 When the current used in these experiments was switched on 

 to the magnet the ionisation was decreased to about 20 °/^ of its 

 original amount, showing that practically all the ^ rays were 

 prevented from entering the chamber by the magnetic field. 



Firstly some measurements were made with no aluminium 

 leaf placed at a. It was found that using a lead plate at c 

 2 mm. thick, the ionisation in the cone / decreased 15 7o. when 

 the current Avas switched on to the magnet ; when the plate was 

 of aluminium 3 mm. thick, the decrease was 23 7o- 



and Madsen, Trans. Roy. Soc. of S. Australia, Vol. xxxii. Jau. 1908. 



