McClelland — Comparison of Capacities in Electrical Work. 173 



Dolezalek type was used, giving a deflection equal to 5300 scale- 

 divisions for 1 volt. 



Observations were taken as before, first with the unknown 

 capacity C joined up to the electrometer and the plate A, and 

 then with the capacity -1 microfarad joined up. 



About 100 grams of uranium nitrate were placed on A (fig. 1), 

 and the following numbers noted ; c denotes the capacity of the 

 Dolezalek electrometer, and some apparatus that was in connexion 

 with it : — 



G+c -1 + e 



50 divisions in 104-5" 100 divisions in 41*8" 



50 divisions in 104-4" 100 divisions in 41-8" 



Mean, 104-45" Mean, 41-8" 



Therefore, 



C+c 208-9 



•1 + c 41-8 



Less than 1 gram of uranium nitrate was now used to 

 determine the ratio of c to a known capacity of '001 micro- 

 farad, giving as follows : — 



c C+-001 



100 divisions in 21-0" 100 divisions in 30-5" 



100 divisions in 21-2" 100 divisions in 30-0" 



m , , c 4- -001 605 



Therefore, __=__. 



These equations give 



c = -0023 microfarad ; 

 C = -5089 microfarad. 



A repetition of this determination gave as before, with smaller 

 capacities, an equally consistent result. 



(cl) A careful experiment was now made to find how small a 

 capacity could be detected and measured by this method. 



To do this a condenser of the following type (fig. 2) was 

 arranged : — 



AB is a long wide tube, 7-90 cms. internal diameter; ah is 

 another tube fixed, as shown, to be coaxial with AB. In ab a third 

 cylinder cd slides, fitting closely into ab, the external diameter of 

 cd being 1-94 cms. AB is joined to earth, and ab (and cd) 



