Absorption by Air of the Beta Rays from Radium C. 9 



A similar process for the ionization due to the 7 rajs 

 gives IV 2 a constant, because o£ the minute absorption which 

 the y rajs undergo, over a range of a few metres in air. 



Electroscopes. 

 One electroscope (20 x 10 X 10 cm.), fig. 1, was built of a 







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2 A77. 



framework of light steel knitting-needles. This was covered 

 with tissue-paper, blackened inside with pencil, weighing 

 •002 gram to the square centimetre, so that the stopping 

 power of the wall was less than that of 2 cm. of air. The 

 framework was suspended, and held firmly bj light steel 

 wires to a wooden frame near the middle of the room, well 

 removed from other bodies. The observer approached the 

 electroscope onlj for a few seconds at the beginning and 

 towards the end of a reading. The microscope could be 

 similarly removed and replaced, but this proved to be 

 unnecessary as the correction was negligible. The mica 

 windows were exceedinglj thin, and the light aluminium 

 leaf sjstem was charged through a trap-door in the paper. 

 Considerable variation in the natural leak occurred at first, 

 and was traced to daylight entering the tissue-paper, and 

 making the sulphur insulator a partial conductor. This 

 trouble was obviated by working in a darkened room, and 

 subsequently by using amber insulators. The effect of 

 daylight on sulphur was first discovered by Bates of 

 Macdonald College, and it has been suggested that sulphur 

 shares to a small degree the well-known property of selenium. 

 In the usual thick-walled electroscope the effect is slight, 

 and may escape notice. 



Another electroscope consisted of a framework of thin 

 knitting-needles mounted on tall light wooden pillars. In 



