146 



J. H. Pratt — Capillary Electrometer. 



such service, first on account of its comparative rapidity in 

 coming to a state of equilibrium ; secondly, because it is prac- 

 tically a dead-beat instrument ; and, thirdly, because the read- 

 ings may be made directly, and show immediately their rela- 

 tion to EM. F. The ease with which the instrument is con- 

 structed puts it within reach of all who have some ability in 

 glass-blowing. 



II. To ascertain whether a current passes when the mercury 

 surface is polarized. 



For this purpose the instrument was connected as before, 

 and, in addition, a reflecting galvanometer and a key were 

 placed in the branch with the electrometer. The deflections 

 were observed by means of a telescope and millimeter scale 

 about four feet from the galvanometer. It was easy to detect 

 a deflection amounting to 0'l mm which, for the distance of the 



falvanometer from the scale meant an exceedingly minute de- 

 ection ; and hence an insignificant current. The arrangement 

 can be seen from figure 3. B is the battery of two Dan- 

 iell's cells. R is a resistance-box of 10,000 ohms. E is the 

 electrometer ; K, the key ; and G, the galvanometer. 



0<? 



~K~ 



IulJ 



The current from the battery was passed through the resist- 

 ance-box. Known fractions of this current made the circuit 

 through the. electrometer and galvanometer. Before the mer- 

 cury reached the position of equilibrium, there was evidence 

 of a current through the galvanometer ; but when once the 

 mercury came to rest, the deflections of the galvanometer 

 ceased. This was true for low potentials and until the E. M. F. 

 reached about 1*4 Dan. Plot II. and Table II., given below, 

 will show the results obtained for hydrogen polarization. The 

 first column gives the potential of the current passed through 

 the galvanometer, and the second gives the final deflection of 

 the galvanometer needle, measured in scale divisions ( mm ) and 

 indicating the strength of the current through the galvanom- 

 eter. 



