Actinium in an Electric Field. 513 



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 n 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 1/2 times. The electrometer was of the Dole- 

 zalek type with a platinum suspension. The needle was kept 

 charged to a potential of 120 volts ; and at this potential the 

 sensitiveness was about L90 mms per volt on a scale a meter 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 connection with the air- 

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

 the potential was applied and the air current regulated to the 

 desired pressure. This method was followed in all the experi- 

 ments and evidently insured a perfectly dry and dust-free 

 stream of emanation flowing into a dust and moisture-free 

 vessel. The electrode and the vessel were thus exposed to a 

 steady flow for a period long enough for the emanation to get 

 into equilibrium with its subsequent products. This period of 

 activation was usually three hours and never less than two and 

 a half hours. 



When equilibrium was established, measurements were taken 

 of the ionization current passing through the vessel, first with 

 the potential (v) used in the particular experiment and then 

 with a standard potential, which was 600 volts in all cases. 

 These ionization currents are designated by I v and I 600 respec- 

 tively. These operations in general disturbed the activity 

 distribution : hence after the ionization readings ample time 

 was allowed for this distribution to be restored. Then the 

 actinium tube was removed, the electric field was switched off 

 and a strong current of dry air allowed to flow for a minute in 

 order to drive out the emanation and deposit particles remain- 

 ing in the vessel. Time was measured from the instant of the 

 removal of the actinium tube. 



The amounts of deposit which settled on the case and the 

 central electrode were measured by the ionization current to 



