Active Deposit of Actinium in an Electric Field. 767 



ionization current readings due to the activity, calculations 

 were made o£ the maximum activity at the time when the 

 deposit was in equilibrium with the emanation, by means of 

 the table given in Appendix II. p. 147 of Makower and 

 Geiger's Practical Measurements in Radioactivity. Ordi- 

 narily three or four readings were taken, and these gave for 

 the maximum activity results which were very consistent and 

 well within the limits of experimental errors. 



The activity on the central electrode was determined by 

 removing it from its holder and suspending it in the vessel 

 B in the same manner as described above, and measuring 

 the ionization current to which it gave rise at definite intervals 

 after the removal of the actinium tube. 



In all measurements of the ionization current, the zero of 

 the electrometer was made the centre of the swing. This 

 was accomplished by the use of the potentiometer arrange- 

 ment mentioned above. 



The amount of emanation in the vessel was varied by 

 varying the distance of the source of emanation from the 

 vessel, keeping the pressure and hence the velocity of the 

 air stream constant. This method was adopted, in preference 

 to changing the stream velocity, for the reason that in this 

 way the relative concentration of the emanation along the 

 vessel would remain constant, as is evident from the form of 



-~x 



the expression, e u , for the concentration at any distance 



x from the bottom. Varying the distance of the source 

 means simply varying the concentration at the bottom of the 

 vessel or shifting the axis of ordinates of an exponential 

 curve along the X-axis. 



The velocity of the air current, which depends upon the 

 pressure and the resistance of the series of tubes and bottles, 

 was roughly measured in the following way: — Exposures 

 were made in the usual manner with different pressures, and 

 the activities on different portions of the electrode were 

 measured separately. This measurement was effected by 

 inserting the wire centrally into a long brass tube of small 

 diameter, which had a small window cut at its middle ex- 

 posing about 6 mm. of the electrode, and placing the tube 

 inside the ionization chamber of an ordinary a-ray electro- 

 scope. A large number of experiments were performed, 

 and it was found that the distribution of activity along the 

 wire decreased exponentially with the distance from the end ; 

 except at the lower extremity, where there w r as an abnormal 

 increase, obviously to be expected w T hen we consider the 

 volume of the emanation to which the end of the electrode 



