99 



Dr. P. B. Perkins : Determination of Periods of 



Experimental Arrangement, 

 The arrangement of the apparatus is shown in fig. 1. B is 

 a cylinder into which the emanation was introduced. The 



Fig. 1. 



5 



490 yons 



To POTENTIOMETER 



/I NO 



1TTERY 



J^Jy>ft«TH 



central electrode was connected to the electroscope E, and to 

 a series of capacities C x and C 2 . The chamber B, 8 cm. in 

 diameter and 14 cm. high, was air-tight and provided with 

 two stopcocks, one leading to the pump and manometer, the 

 other to the source of emanation. 



By slightly exhausting the chamber the emanation from 

 radio-thorium or actinium, wrapped in filter-paper and en- 

 closed in glass tubes placed at A, could bo quickly swept into 

 the chamber, by air entering through the drying-tube D. 

 Plugs of cotton- and glass-wool W and W l5 respectively, were 

 inserted to prevent " old " active deposit from entering the 

 chamber along with the emanation. The electrode was about 

 2 mm. in diameter, and passing through an earthed guard- 

 ring, reached to within 2 cm. of the top of the chamber, and 

 was joined through an insulated brass cup C to the gold-leaf 

 of a Wilson-Kaye electroscope E. A thin copper window K 

 permitted ft rays to enter the chamber from a source above 

 for standardization purposes The chamber was insulated and 

 joined through a water resistance to a battery of 490 volts. 

 The capacity Q 1 was a standard mica condenser, divided into 

 parts marked *1, *02, '005, and -001 microfarad respectively, 

 and Co was a small cylindrical capacity of approximately 

 470 cm. Kevs K : earth connected and K 2 to which the 



