544 Lucian — Distribution of the Active Deposit of 



which they gave rise, with an applied potential of ■+• 200 volts 

 on the case. Although this potential was not sufficient to 

 afford saturation current, later experiments showed that the 

 ratio of the activities thus measured was the same as that 

 obtained by using higher potentials. To measure the case 

 activity, the ebonite plug P containing the central electrode 

 was removed, care being taken that the electrode did not touch 

 the sides of the vessel, the gauze bottom was removed and a 

 clean flat bottom was screwed on, to avoid spurious effects due 

 to deposit particles collected by the cotton wool. Then a fresh 

 electrode held in a stiff brass rod, without the ebonite plug, was 

 introduced into the vessel and held as usual by the brass clip 

 H ; and a series of readings of ionization current were taken 

 at definite intervals after the zero of time. It is to be noticed 

 that by avoiding the use of ebonite the ionization current could 

 be measured with great precision. As the case activities were 

 in general small, their exact value was of relatively greater 

 importance than those of the central electrode ; hence a greater 

 number of readings were taken with and without the capacity 

 C in order to check the results. It might be mentioned that 

 almost all the readings were taken with the capacity added to 

 the system. From these ionization current readings due to the 

 activity, calculations were made of the maximum activity at 

 the time when the deposit was in equilibrium with the emana- 

 tion, by means of the table given in Appendix IT, p. 147 of 

 Makower and Geiger's Practical Measurements in Radioactiv- 

 ity. Ordinarily three or four readings were taken and these 

 gave for the maximum activity results which were very con- 

 sistent 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 ioniza- 

 tion 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 center of the swing. This was 

 accomplished by the use of the potentiometer arrangement 

 mentioned above. 



The amount of emanation in the vessel was varied by vary- 

 ing 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 the expression, 



e v , for the concentration at any distance x from the hot- 



