produced in a gas by <y rays. 



171 



propagation of the rays, there was a diffuse pencil of /3 rays 

 projected into the chamber from its lower side. If the ionisation 

 is produced by the /3 rays from the walls of the chamber, the 

 ionisation ought to be principally due to this beam, and should 

 therefore decrease considerably when a strong magnetic field is 

 applied. It was found, however, that a magnetic field produces 

 little change in the amount of ionisation. Thus in a particular 

 case the ionisation in a sheet lead chamber 5 5 cm. high, 5*5 cm. 

 broad, and 7'5 cm. long, was 1895 in arbitrary units. When a 

 field of over 2000 units was applied, which was sufficient to bend 

 the rays having the same velocity as the penetrating /3 rays from 

 radium into a circle of radius less than 8 mm., the ionisation 

 current decreased to 1645, or about 12 7o- When a chamber 

 7 cm. high, 4 cm. long, and 3 cm. broad was used, the current 

 changed from 1560 to 1475, when the magnetic field was applied, 

 a decrease of about 6 °/^. 



If all the ionisation in the gas was produced by the yS rays 

 from the walls of the chambers, the ionisation should obviously 

 have decreased to a greater extent in each case, although a part 

 of the path of each ray when deflected was still contained by 

 the chamber, and consequently produced ionisation. 



Since part of the leak in this experiment was always due 

 to the /3 rays from the walls of the ionisation chamber, it was 

 thought desirable to carry out the experiment in a somewhat 

 different form. Figure 2 gives a diagram of the apparatus used. 



A is a cylindrical ionisation chamber 19 cm. long, and 8 cm. in 

 diameter, to which the tube ab 6*5 cm. long and 3'5 cm. in diameter 

 was co-axially attached. The chamber was placed so that the 

 tube ab was between the poles of an electro-magnet. The end 

 a of the tube was closed in one set of experiments by a sheet 

 of thin aluminium leaf equivalent in mass to a layer of air "7 cm. 

 thick, the end b of the tube being closed with a metal plate c. 



