366 Mr. Briggs on Distribution of Active Deposits of 



A ciean plate having been placed on the ebonite plate 

 and charged negatively, a second plate was exposed for 

 ten seconds to radium emanation and quickly placed opposite 

 the first, the action o£ a spring immediately connecting it 

 to the positive pole of the battery (720 volts). Dry filtered 

 air was drawn through the apparatus for about one minute, 

 the final pressure being about 15 cm. The time that elapsed 

 between removing the active plate from the emanation and 

 putting it in position was usually about five seconds. In this 

 case 0*033 of the RaA has decayed before the active plate is 

 in position. At the end of 27 minutes the two plates were 

 removed, and their activities compared by means of a 

 Dolezalek electrometer of sensitivity 2000 mm. per volt. 

 The results obtained, after correcting for the initial decay 

 of the RaA, for the efficiency of recoil in air of RaB from 

 plates of brass, steel, and zinc, cleaned with emery paper, 

 were respectively 81'7, 73, and 73"9 per cent. Tim values 

 given by different readings remained constant to within 

 about 2 per cent. 



The Percentage of RaB positively charged in Air. 



The percentage of R iB initially positively charged in air 

 was measured by modifying the method described above for 

 comparing gases with air. It was arranged that the RaB 

 atoms leaving plate (1) during the first 5 minutes' exposure 

 should be received by a disk fastened to plate (2), so that it 

 was separated from plate (1) by a distance of 0*06 mm. 

 Mica on the edge of the disk prevented contact with (1). 

 A potential of 160 volts was maintained between plate (1) 

 and the disk, the latter being negative. Plate (2) and the 

 disk were placed in position first, and the pump started. 

 Plate (1), with its deposit of RaA, was then placed in 

 position, and the stop-watch started. The pump was kept 

 running throughout the exposure and rapidly produced a 

 vacuum of 0'0l mm. At the end of the five minutes air 

 was admitted and the disk quickly removed. An exposure 

 was then made with plate (3) opposite plate (I) from the 

 6th to the 11th minute in air at 15 c:n. pressure at the 

 original distance of 1*5 cm., using 720 volts. Since the 

 range of recoil of RaB at 0'01 mm. pressure is several 

 metres, all the RaB atoms shot off from the plate should 

 be collected during the first exposure. The mean result 

 found by this method w T as 93 + 0*6 for the percentage of 

 R iB recoil atoms initially charged in air. By multiplying 

 the values given in column 2 by 0'93, the corresponding 



