554 Mr. J. J. E. Durack on the Specific Ionization 



If V 2 be kept constantly =0, and V x be small in comparison 

 with V 3 , this formula takes the usual form 



Now it has been shown by Hopkinson* that the apparent 

 capacity of a pair of quadrants (say the pair 1) when the 

 needle is charged to a potential V 3 is equal to 



V* 2 (A) 



gu + 



dq n 2 



d'e 



Suppose a quantity of electricity Q l given to the pair of 

 quadrants 1, the potential Y 1 will be 



and for the formula A we may write 



Hence 0/Q, (which is the sensibility for current) has a maxi- 

 mum value for a certain value of V 3 , viz. : — 



- V^llT^ 



dqu \ 

 old ) 



When high sensibility for leaks is an important factor the 

 needle should be charged to this potential, and even when 

 high sensibility is not required it is advisable to work with 

 the electrometer at its maximum sensibility, for then small 

 changes in V 3 make no change in 0/Q^ 



The value of V 3 for which 6/Q l is a maximum is easily 

 found by measuring the capacity of a pair of quadrants and 

 rinding "for what value of V 3 this capacity is equal to 2q lv 

 i. e., twice the capacity when V 3 = 0. With my electrometer 

 this was the case when V 3 was equal to 60 volts. The 

 observations from which a was to be deduced were made as 

 follows : — The plate PP was connected to one pole of a battery 

 of cells (say the positive pole), the other pole being to earth, 

 a small P.D. was set up initially between the two pairs of 

 quadrants, the insulated pair being initially negative with 

 respect to the earthed pair if the leak to be measured was 

 positive, and positive if the leak was negative; thus the needle 

 travelled through its zero position in taking a reading, which 

 was done by finding the time required for the light-spot to 

 pass over a certain number of divisions on the scale, these 

 divisions being half on one side and half on the other side of 

 the zero. 



By reading in this way the effect of changes in capacity 

 * Phil. Mag. ser. 5, vol. xix. 



