Activity of freshly-formed Radium Emanation. 715 



this period the testing vessels -were kept charged, the central 

 wires being earthed so as not to cause variations in the 

 distribution of the deposit. Before the commencement of 

 an experiment the strip was thoroughly deemanated by 

 heating, the emanation which had accumulated on it being 

 swept away down a pipe leading out the window. 



The testing vessels were copper cylinders about 3*2 cm. 

 in diameter and 180 c.c. volume mounted vertically side by 

 side. They were connected together and charged to a 

 negative potential of 143 volts by means of a battery of small 

 dry cells. The central wires of the testing vessels passed 

 through earthed guard tubes in the rubber stoppers which 

 closed the upper ends of the vessels, being insulated from 

 the guard tubes by means of sulphur. Either of these wires 

 could be connected to the electrometer or earthed by means 

 of two small mercury cups inside an earthed metal vessel 

 fixed just above the testing vessels. Except when connected 

 to the isolated quadrant of the electrometer during a test, 

 the wires were always earthed. The air current entered the 

 vessels by side tubes near the top, and left them by vertical 

 tubes in the centre of the conical bottoms. The inlet tubes 

 were tangential so that the air current would flow down the 

 vessels in a helical path instead of forming irregular eddies. 



A Dolezalek electrometer was used with a fairly coarse 

 quartz suspension such that the deflexion per volt was about 

 160 scale-divisions, the needle being charged to a potential 

 of 143 volts. The earthing key consisted of an iron wire 

 dipping into the mercury cup used for connecting the testing- 

 vessels. 



As it was necessary to prevent active deposit and also ions 

 formed during the passage of the emanation through the 

 long tube from entering the second testing vessel, this was 

 guarded by a second cotton-wool plug. It was also necessary 

 that the air in the two testing vessels should be at approxi- 

 mately the same temperature as otherwise the emanation 

 would traverse the two at unequal rates. Accordingly , 

 between the cotton-wool plugs and the testing vessels, the air 

 flowed through two straight brass tubes 38 cm. long placed 

 side by side in the same water-jacket. Inside these tubes, 

 which were 9'2 mm. internal diameter, were mounted two 

 smaller tubes 6*45 mm. external diameter blocked at each 

 •end, so that the air current traversed the narrow annular 

 space between the tubes, thus encountering a large cooling 

 surface in a small volume. The inner tubes were insulated 

 from the outer ones and charged to the same potential as 

 the vessels, thus providing an electric field of about 1040 volts 



