534 Mr. N. R. Campbell on Radiation 



was measured by the electroscope. The surface of the wire 

 forming the electrode was so small that its influence was 

 negligible compared with that of the walls of the vessels. 



The front wall could be removed in order to adjust the 

 sliding side ; it is supposed to be removed in the "figure to 

 show the interior arrangement. The dimension of the sliding 

 side was about 33 x 23 cms., differing slightly in different 

 boxes ; the range was some 22 cms. 



The following materials were used as coverings for the 

 walls : — 



Lead sheet *1 cm. thick. 



Tinfoil _ -0013 cm. „ 



Aluminium *05 cm. „ 



Zinc '08 cm. „ 



Platinum '01cm.* „ 



Carbon (wood coated with pure graphite). 



The results of the experiments are expressed in the curves 

 of fig. 1, Plate VII. t Each point marked on a curve repre- 

 sents the mean of at least fifteen observations — usually a far 

 greater number. The plotting of each curve occupied from 

 seven to ten days. 



§ 5. Before proceeding to the discussion of the curves some 

 attention should be given to a difficulty which seriously 

 obstructed the progress of the work. This difficulty is the 

 variation which was found to occur in the value of the leak 

 through a given volume. 



The variations were of two kinds : — 



(1) A slight continuous and continual change over a range 

 of 15 or 20 per cent. No serious trouble arose from this 

 cause ; by taking the mean of some 25 readings a value could 

 always be obtained which could be reproduced to within 4 or 

 5 per cent, at any subsequent time by replacing the sliding 

 side in the same position — so long as changes of* this kind only 

 had occurred in the interval. 



(2) Sudden discontinuous changes or "jumps," when the 

 value of the leak might be altered by as much as 50 per cent. 

 These were far more troublesome. They occurred on an 

 average once in 2 or 3 days, and affected in the same sense 

 the values all over the range. The leak could not be brought 

 back to its original value by blowing in fresh air or cleaning 

 the surface of the walls ; the occurrence of the variations 



* For the loan of the considerable quantity of platinum required, I 

 am indebted to the generosity cf Messrs. Johnson, Matthey & Co. 



t The curve for platinum is so nearly coincident with that for 

 aluminium that it has not been drawn. 



