596 



Mr. J. Chadwick on the 7 Rays 



to a plate of any given material placed at P. The plates 

 used were about 3J cm. square and sufficiently thick to stop 

 all fi rays. 





< 



The ionization in the chamber A was measured by a 

 modified form of the balance method of Rutherford and 

 Chadwick *. The ionization current in A due to the 7 rays 

 was balanced against an equal and opposite ionization current 

 due to a film of uranium oxide. As it was impossible to 

 move either the emanation tube and magnet, &c, or the 

 ionization chamber, it was arranged that the current due to 

 the 7 rays from the source E was less than that due to the 

 uranium source, and a balance was obtained by adjusting 

 the position of a radium tube R along the scale S. 



The method of observation was as follows : — A plate of 

 any material was placed at P and the position of the radium 

 R adjusted till a balance was obtained. The magnetic field 

 was then excited, and it was found that to obtain a balance 

 the distance of the radium from A must be increased. In 

 the majority of the experiments the change in the balance 

 position corresponded to a change of about 20 per cent, of the 

 ionization due to R, that is, the ionization produced in A by 

 the excited 7 rays from P was one-fifth of the ionization due 

 to R when in the first position. 



It has been shown in a previous paper that the intensity 



of the 7 rays from a radium-source at a distance r is very 



e~v- r 

 nearly proportional to — g-, where fx is the coefficient of 



absorption of the 7 rays by the air. 



If r x is the distance from the centre of A of the position 

 of balance of the radium R without the magnetic field, r a 

 the corresponding distance with the magnetic field, then 

 * Rutherford & Chadwick, Proc. Phys. Soc, April, 1912. 



