Active Deposit of Radium in Electric Fields. 545 



Of the anode activity, therefore, only about 6 per cent, can 

 be attributed to matter recoiling directly on to the electrode 

 (diam. 3*23 mm.), and at most of the order of 6 per cent, to 

 emauation absorbed by the electrode. A similar calculation 

 in the case of the activity on the inner surface of the chamber 

 shows that there the active matter is several times greater 

 than can be accounted for by recoil. 



The Electrical charge carried by the Anode deposit. 



There exists the possibility that some, if not all, of the 

 anode activity is negative!}' charged deposit. If it consist 

 solely of negatively charged matter, the anode activity will, 

 be independent of the size of the anode, provided we do not 

 alter other conditions, whereas, if the matter consisted solely 

 of uncharged deposit, the activity of the anode must be 

 some function of its area. If it consist partly of charged 

 and partly of uncharged active deposit, then we can deter- 

 mine the proportions of each provided we can find the 

 limiting value to which the anode activity tends when the 

 area of its surface becomes infmitesimally small. This limit 

 obviously gives the amount of negatively charged deposit. 



A direct determination of the variation of anode activity 

 with the area of the electrodes was therefore undertaken. 

 A fixed potential was employed, the cases of both chambers 

 were at —960 volts, and both electrodes were earthed. The 

 .activities collected by rods of various diameters were com- 

 pared with that collected by the electrode (diam. 2'00 mm.) 

 used in the initial experiments. In altering the size of the 

 electrode we alter the quantity of emanation in the chamber, 

 and also the mean time taken for the active deposit to travel 

 from the gas on to the electrode. The latter determines 

 the amount of recombination of the active deposit which 

 occurs. The volume of emanation employed is very nearly 

 proportional to 



7r/(7, 2 -a 2 ), 



where " a " is the radius of the central rod, " I " its length, 

 and " b " the radius of the chamber. The mean time of 

 passage T from the gas to the electrodes is given by 



T =TV^ W ) ,0 4 



where V is the potential difference between the rod and the 

 case and k is the mobility of: the active deposit at atmo- 

 spheric pressure. The changes introduced into these quanti- 

 ties when " a " is changed depend mainly on the factor 

 (Jr — a 2 ). The variation in this quantity from the smallest 

 Phil. Maq. S. 6. Vol. 28. No. 166. Oct. 1914. 2 N 



