302 



Mr. C. S. Wright on Variations in the 



Before making the final measurements in these comparisons 

 preliminary observations were made to ascertain what voltage 

 it was necessary to apply to a receiver in order to obtain 

 saturation currents. A set of the different voltages applied 

 and the corresponding values of the currents obtained with a 

 lead cylinder of comparatively high activity, but having 

 dimensions the same as cylinder No. 1 mentioned above, are 

 given in Table IV., and a curve to represent them in fig. 5 

 (PL V.). From the latter it may be seen that a potential of 

 60 volts gave a reading well over the knee of the curve, and 

 as the voltage applied in all the determinations lay between 

 80 and 250 volts, it is clear that saturation currents were 

 always obtained. 



Table IV. 



Voltage on receiver 



" q "=No. of ions 



(positive). 



per c.e. per tec. 



2 



136 



4 



21-3 



6 



26-2 



8 



29-3 



11-2 



33-3 



17-7 



37-8 



340 



40-8 



47-0 



41-6 



63-3 



42-9 



81-6 



429 



163-2 



433 



251-2 



44-2 



3. Some Special Observations with Lead Cylinders. 



It will be remembered that Prof. McLennan, in his inves- 

 tigation of last year, drew the conclusion that ordinary 

 commercial lead, in so far as he investigated it, contained in 

 general an active impurity. Such being the case, one should 

 expect a certain falling off in the course of time of the 

 ionization in a lead receiver, due to the decay of the activity. 

 It was decided then to determine again the conductivity of 

 air in certain lead receivers used by him in June last in his 

 investigation, to see whether any decrease from the former 

 values could be noted. Lead cylinders described as numbers 

 1, 2, and 3 in Prof. McLennan' s paper were used for this 

 purpose,, and the values of the ionization for each cylinder 

 determined with both the electroscope and the electrometer. 

 These numbers, which are given in Table V., show a con- 

 siderable decrease in the value of " q " from the values for 

 each receiver obtained by Prof. McLennan. 



