Actinium in an Electric Field. 545 



torn. Yarying the distance of the source means simply vary- 

 ing 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 exposing about 6 mms of 

 the electrode, and placing the tube inside the ionization 

 chamber of an ordinary a-ray electroscope. A large number 

 of experiments were performed and it was found that the dis- 

 tribution of activity along the wire decreased exponentially 

 with the distance from the end ; except at the lower extremity 

 where there was an abnormal increase, obviously to be expected 

 when we consider the volume of the emanation to which the 

 end of the electrode was exposed. Curves were plotted for each 

 pressure and from these the distance x, in which the activity 

 fell to half- value, was measured and the velocity v calculated, 

 taking the half-value time to be 3*9 sees. It must be remem- 

 bered that these determinations of velocity for a given pressure 

 are not absolute, and that any change in the resistance of the 

 line will change the velocity of the flow. This was noticed 

 constantly during the course of the investigation. 



The pressure of the air-current finally chosen was about 

 14-0 cms of H 2 S0 4 . With this pressure the velocity obtained 

 was about 5 mms per sec. and the concentration of the emanation 

 fell to less than one per cent in a distance of about 15 cms , which 

 is more than one-half the height of the vessel employed. It 

 will be seen that with this velocity more than one-half of the 

 emanation would be found above the first few mms. of the 

 central electrode, the only places where any irregularities may 

 be expected. Thus the velocity chosen gives us a fair degree 

 of approximation to the ideal case where the emanation may 

 be uniformly distributed in the vessel and the deposit made 

 evenly along the length of the electrodes. 



As is to be expected, experiments with lower pressures, such 

 as 9 cms and 5 cms of Ii 2 S0 4 gave smaller values of the percent- 

 age of cathode activity, due to edge-effect and other causes 

 which helped the case to get more than its share of the activity. 



Mention might be made here of one or two experiments in 

 which it was found that the distribution of the activity was 

 perceptibly affected (the cathode percentage activity being 

 increased) in the case where the flow was asymmetrical with 



