produced in a gas hy 7 rays. 



173 



An aluminium leaf was then placed at a, the ionisation in the 

 tube ah being now excluded from the leak. The decrease obtained 

 with a lead radiator when the magnetic field was applied was 

 23 7o> ^^d 31 °/q with the aluminium radiator. 



The secondary j3 radiation from the thin zinc wall d of the 

 ionisation chamber produced by the cone of 7 rays / was much 

 smaller than that from the plate c because the zinc wall was much 

 thinner than the plate, and also because the returned /S radiation 

 from a plate is much smaller* than the radiation which is 

 propagated in the same direction as the 7 rays, the difference 

 being the greater the thinner the plate. The figures obtained 



n 



D 



Fig. 3. 



show therefore that more than 50 °/^ of the ionisation in the cone 

 / is due to the direct action of the 7 rays on the air. 



Some further measurements were carried out with a slight 

 modification of the foregoing experiment. A diagram of the 

 modified apparatus is shown in figure 3. A is an ionisation 

 chamber lOS cm. long, 10*4 cm. broad, and 7 cm. deep, of which 

 the upper and lower sides consisted of thin tightly stretched 



* Trans. Roy. Soc. of S. Australia, 1908, Pt. 1. 



