Deposit of Radium in an Electric Field. 487 



In practically all our experiments the activities were meas- 

 ured by observing the ionization currents to which they gave 

 rise in air under the ordinary conditions when a potential 

 of +160 volts was applied to the vessel employed. Although 

 this potential was not sufficient to saturate the currents, sub- 

 sequent experiments showed that the ratio of the two activities 

 as thus measured was not altered by using a larger voltage. 



A typical set of readings is given later. 



3. Experiments toith Small Applied Potentials. 



It is well known that for moderately high potentials the 

 activity is concentrated to a very large extent on the negative 

 electrode in an electric held ; Schmidt* has shown that when 

 small potentials are employed the cathode activity decreases 

 in a manner analogous to the diminution in electric current 

 which follows as a result of recombination of ions ; in fact, he 

 came to the conclusion that the active deposit particles behave 

 in a manner similar to the positive ions. The first set of 

 experiments tried in the present series had as object to find 

 whether the amount of activity that Was deposited on the 

 cathode when a small positive potential was applied to the 

 case could be altered when the ionization in the gas was so 

 increased by means of Rontgen rays that the electric current 

 through the gas was at least as great as if a large potential had 

 been applied without any extraneous source of ionization. 



In this set of experiments the rods were exposed as 

 cathodes to the process of activation for a period of 10 min- 

 utes, both with and without the application of Rontgen rays, 

 the testing vessel containing air at 1 atmosphere ; the activ- 

 ities were measured at various intervals after removal, and 

 were then directly compared. Preliminary trials which were 

 made showed that in the process of removing one rod and sub- 

 stituting another less than 2 percent of the emanation escaped. 



A result of one set of measurements is given below : 



With + 4 volts on case and 10 minutes exposure, activity of 

 cathode 10 minutes after removal is represented by an electro- 

 meter rate of l'l mm per sec. 



With same applied potential and strong Rontgen rays pass- 

 ing through the vessel during the whole time of exposure, the 

 corresponding rate was '88. 



As a rough idea of the relative currents involved the follow- 

 ing figures are given ; in these instances the rates were deter- 

 mined with the extra capacity added to the system. 



Ionization current immediately after introduction of ema- 

 nation and with +160 volts on case : 33 mm per sec. : 

 with 4 volts rate was 1*05 ; 

 with 4 volts and Rontgen rays rate was 5*21. 



* Schmidt, Phys. Zeitschr., ix, p. 184, 1908. 



