528 McCoy and Ashman — (Jrano-Uranic Oxide. 



was, therefore, — =36*83 E.S.U. It is considered 



that when the inner sphere of the condenser is placed in posi- 

 tion the calculated capacity, 38*83 E.S.U., is added to that of 

 the system when this sphere is absent, but all else arranged as 

 shown in the figure. Calling the capacity of the electroscope 

 alone c 1 and with the condenser c a , then the ionization current 



i=^±J.=tli-i where p l andjp 2 are the potential differences cor- 



responding to the two ends of the scale and t x and t t are the times 

 of discharge for the same film for the changes of potential p 1 

 and p 2 respectively. Therefore 



hPi — ^P* 



The quantity c 2 — c 1 is the capacity of the condenser = 36*83 

 E.S.U. 



We used film No. 30, Table 3, placed at a distance of 3*6 cm 

 below the electrode of the gold-leaf system. Experiment 

 showed that this distance was entirely sufficient for the pro- 

 duction of the maximum ionization current ; at a distance of 

 4*5 cm or more the current was slightly smaller, owing, of course, 

 to partial recombination of the ions. 



As the closely agreeing means of several determinations, the 

 times of discharge of the electroscope with and without the 

 condenser were 243*9 and 420*9 sec. respectively, for the 

 uncovered film. The times when the a-rays and 9*2 per cent 

 of the /3-rays were cut ofi by a sheet of aluminium 0*0043 cm 

 thick were 10,860 and 12,435 sec. respectively.* From these 

 data it follows that the times of discharge for the a-rays alone 

 were 249*9 and 436*4 sec. respectively. The fall of potential 

 across the scale was 103*05 volts without the capacity and 

 102*63 volts with the capacity, the measurements being made 

 with an electrostatic voltmeter. From these data it is found 

 that o t = 48*84 E.S.U. and c, = 36*83+48*84 = 85*67 E.S.U. 



Therefore the a-ray ionization current i == - — \ * =2*238 X 



10" 11 amp. Film No. 30 is T*0168 cm in diameter ; its area= 

 38*67 sq. cm. Therefore the current per sq. cm. = 5*79xl0~ 13 

 amp. The total activity of 1 g. of uraniumf is equal to that 

 of 796 sq. cm. of a thick film of U 3 8 . Therefore the total 

 a-ray ionization current of 1 g. of uranium =4*61 X 10 -10 amp. 



* These times represent the 90*8 per cent of the /3-ray leak plus the natural 

 air leak and the leak across the insulation. The latter is somewhat greater 

 when the condenser is attached to the electroscope ; this makes the time 

 12,435 sec. shorter than would otherwise be expected. 



f McCoy and Eoss, loc. cit. 



