Active Deposit of Actinium in an Electric Field. 765 



spring-brass clip slipped over the brass rod to connect the 

 central electrode with the electrometer system. E is a 

 guard tube, which is always kept earthed to prevent any 

 leakage over the surface of the insnlators. F is a brass ring 

 which was employed in order to remove the ebonite plug P 

 and the electrode without the necessity of handling the 

 insulation. G is a piece of fine-mesh gauze placed over the 

 cotton-wool in order to prevent an abrupt change in the re- 

 gularity of the electric field at the bottom of the vessel and 

 also to stop cotton-woo] particles being blown into the test- 

 vessel, b is a detachable brass piece placed centrally over 

 the gauze in order to make the flow symmetrical, as much 

 as possible, with respect to the case and the central electrode. 

 The vessel was supported on the side by spring-brass clips 

 (not shown in the diagram) which were mounted on ebonite 

 and were connected to a series of high-potential accumulators 

 through the water resistance H and the triple key M. 



The wire B in the figure goes to a second vessel, of con- 

 struction identical with A, except for the bottom, which was 

 a flat brass piece in this case. KR'D is a potentiometer 

 arrangement which was employed whenever it was necessary 

 to charge the quadrant system to any desired potential. It 

 might be worth mentioning that the key K was connected with 

 the double-lever system in such a way that by closing K 

 either way simultaneous contact was made at L with the 

 electrometer lead ; thus the use of two separate keys was 

 avoided. 



The usual electrometer key-system is represented by K 1} 

 E, and K 2 . C is a capacity of tinfoil and mica, which when 

 added to the electrometer system would increase the total 

 capacity of the system 48^ times. The electrometer was of 

 the Dolezalek type with a platinum suspension. The needle 

 was kept charged to a potential of 120 volts ; and at this 

 potential the sensitiveness was about 190 mm. per volt on a 

 scale a metre away. The leads, the keys, the capacity, and 

 the electrometer were carefully screened from all electrical 

 disturbances. 



The potentials used, varying up to 1700 volts, were obtained 

 from a battery of high-potential cells. 



The method of procedure in general was as follows: — 



The air current was first established ; the vessel was con- 

 nected to an air-pump and was exhausted to a pressure of a 

 few mms. of mercury, the outer surface being simultaneously 

 heated moderately in order to expel traces of residual gas. 

 The vessel was then quickly put in connexion with the air 

 current, which was made to pass through the actinium tube ; 



