Deposit of Radium in an Electric Field, 439 



tends to reach the value of the cathode activity with 

 the increase o£ the quantity of emanation. In the light 

 of the results given above, this indicates that in a vessel con- 

 taining large quantities of emanation practically all the recoil 

 atoms are neutralized owing to the intense ionization in the 

 vessel, and are carried to the electrodes by the electric 

 wind. 



6. It follows from the results given in the previous section 

 that the amount of activity acquired by an electrode when 

 exposed to a large quantity of emanation depends upon the 

 direction of the electric wind in the exposure vessel. It is 

 easy to see, however, that the gradual increase of the activity 

 deposited on the electrode during the exposure involves an 

 appreciable change in the direction of the electric wind. 

 Thus, at the beginning of the exposure the density of ionization 

 is uniform throughout the volume of the vessel, while when 

 the electrode becomes active the density of ionization decreases 

 with the distance from the electrode. In other words, the 

 activity deposited on the electrode serves as an additional 

 source of ionization in the experiments described above, with 

 the result that the amount of active matter carried to the 

 electrode per unit time diminishes with the increase of the 

 activity on the electrode. Since the ionization due to this 

 activity is not large compared with that produced by the 

 emanation, this effect, however, must be small in the present 

 case. 



It seemed of interest to ascertain whether this effect is 

 large enough to disturb appreciably the law of accumulation 

 of the active matter on the electrode as given by theory. It 

 was found convenient for this purpose to investigate the 

 quantity of radium A present on the electrode as a function 

 of the time of exposure. The electrode was exposed to the 

 emanation for a given time, and the quantity of radium A 

 accumulated on it was measured by the amount of radium B 

 given up by the recoil from the active surface of the electrode 

 during a definite interval of time. The positively charged 

 recoil atoms of radium B were collected on a plate 1 mm. 

 distant from the active surface of the electrode, an intense 

 electric field (about 10,000 volts per cm.) being established 

 between the plates in order to prevent recombination of the 

 radioactive ions. Under these conditions the amount of 

 radium B collected on the plate is proportional to the 

 quantity of radium A present on the electrode and to a 

 certain coefficient known as the efficiency of recoil, which 

 depends upon the physical conditions of the active surface. 

 It w r as found that in the case of aluminium foil this coefficient 



