42 Mr. Norman Campbell 



Hence 



' V } + 'I 2 4. - 8 + - n — - n ^ n 



f2Jj «^j tZ"j[ i3Jj A^ <2?i 



That is 



X 



t2?l H~ 4^2 ~J~ "^3 r • ■ '^"w A? 



Hence in a series of substances in radioactive equilibrium 

 the ratio of the number of atoms of the nth substance to 

 those of it and all the preceding substances in coexistence 

 with it is equal to the ratio of the average life of the nth 

 substance to that of the first substance present in the series. 



Higli School, Dundee. 



VI. A Note on a Method of Determining Capacities in 

 Measurements of Ionization. By Norman Campbell*. 



THE ordinary method of determining the capacity of the 

 ionization vessels and the measuring instrument used 

 in observations on ionization consists in comparing the rate 

 of rise of potential of the electrode system for a constant 

 current (1) when a known capacity is put in parallel with 

 the electrode, and (2) when that capacity is absent. The 

 advantages of the method lie in the simplicity of the appa- 

 ratus required ; its disadvantages are connected with the 

 choice of the standard capacity. Standard condensers of not 

 less than *01 m.f. are easily procured, but their capacity is 

 very great compared with that of the usual measuring 

 systems, which is usually less than *0005 m.f. If they are 

 used, either the time for which the rise of potential is 

 observed in (1) must be so long that difficulties are likely to 

 arise from defective insulation, or the time for which the 

 rise is observed in (2) must be so short that it cannot be 

 measured with accuracy. On the other hand, accurate and 

 w T ell-insulated standard condensers, the capacity of which is 

 of the same order as that of the measuring instrument, 

 are not easily procured, and the (unknown) capacity of the 

 conductors used to connect them to the measuring system 

 may be too large a fraction of their capacity to be safely 

 neglected. 



The following method, which appears not to be known 

 generally, is free from these disadvantages. Though there 

 is nothing new in its principle, which is sufficiently obvious, 

 it may be worth while to draw attention to it. 

 * Communicated by the Author. 



