Section- III, 1920 [73] Trans. R.S.C. 



The Capacity of the Capillary Electrometer 

 By A. L. Clark, Ph.D., F.R.S.C. 



(Read May Meeting, 1920) 



In spite of the fact that the capillary electrometer has been the 

 subject of study ever since the pioneer work of Lippmann\ the question 

 of the cause of the large capacity and its possible variation with the 

 potential difference cannot be said to have been answered satisfactorily. 

 The controversy over the explanation of electro-capillary phenomena 

 and the existence of an electric double layer at the boundary between a 

 metal and an electrolyte or a gas is still open to contribution and the 

 importance of the question makes advisable the publication of the 

 following investigation. 



If the large capacity of the electrometer is due to a double layer 

 such as conceived by Helmholtz,"^ it might be expected that the 

 properties of this double layer would depend on the difference of 

 potential of the two sides and the capacity^ might be a function of 

 this potential difference. Contrary to the explicit statement of 

 Burch* that the capacity is independent of the difference of potential, 

 preliminary observations showed that the capacity is a function of the 

 potential difference. Also, contrary to his statement of the case, the 

 capacity is very different when the direction of the applied potential is 

 reversed. As ordinarily used, the mercury of the electrometer is the 

 cathode. Otherwise the mercury becomes fouled after a short time 

 and sticking results. Later study of the variations of capacity have 

 confirmed the results obtained earlier, and the following paper is an 

 account of the work carried out in the summer of 1917. 



Apparatus 



The form of electrometer used is essentially the same as that used 

 by Burch with a few modifications for convenience in handling and for 

 changing capillaries. The tubing was of the soft, soda-glass variety 

 selected to be free from irrgeularities. It was washed with hot aqua- 

 regia, water, hot potassium hydroxide and distilled water and finally 



» Lippmann, Pogg. Ann. 149,546, 1873; VVied. Ann. II, 320, 1880. Compt. 

 rend. 76, 1407, 1873; 95, 686, 1892. Ann. de Chem. et Phys. (5), 494, 1877; Jour. 

 de Phys. (2) 2, 116, 1883. 



2 Helmholtz. Wied. Ann. 7, 337, 1879; 16, 30, 1882. 



' The word capacity is used here in the sense of apparent capacity. 



* Burch, Electrician (London). July 17, 27, 31, Aug. 7, 1897. 



Sec. III. Sig. 6 



