﻿190 



Mr. A. S. Eve on the Radioactive Mattel 



or wire cage was suspended. A rod, one centimetre in 

 diameter , was first used, but difficulties arose in obtaining a 

 saturation current. A cylindrical cage of 

 clean copper wire about 40 cms. long and 

 7 cms. in diameter, was therefore substi- 

 tuted for the rod. Each vessel had then 

 a capacity of about 11 electrostatic units. 



The electroscopes could be interchanged 

 when required, and insulated from their 

 respective vessels by mica sheets. This 

 most convenient method was recommended 

 to me by Professor Rutherford. Bragg 

 has shown the difficulty of securing com- 

 plete saturation in the case of weak cur- 

 rents due to feeble radiation. But, by 

 the arrangement described, the lower 

 vessels could be earthed, the case of the 

 electroscope maintained at a high potential 

 by a constant battery, and the w ire cage 

 and goid-lea£ charged to a yet higher 

 potential. In this way saturation may 

 be ensured between the large vessel and 

 wire cage, whilst the gold-leaf is deflected 

 to a convenient amount due to a moderate difference of po- 

 tential between the gold-leaf and the case of the electroscope. 



After deducting the corrections for natural leaks the results 

 were as follows : — 



Table I. 



Substances. 



Thickness in mm. 



Ions per c.c. per sec. 



Lead 



1-6 

 2-7 



•4 



•57 



•57 



•6 



•4 



550 

 482 

 448 

 351 

 345 

 320 

 297 



Lead within lead 



Tinned iron 



Zinc 



Copper 









There is clear evidence of much secondary radiation from 

 the heavier metals. In the lead vessel 45 per cent, of the 

 ions must be attributed to this cause, for there is no other 

 reason why the ions should be more numerous in the lead 

 than in the aluminium. It will be noted that tinned iron 

 takes the place due to tin rather than iron. The order of 

 ionization need not necessarily be that of atomic weight, 



