J. H. Pratt — Capillary Electrometer. 145 



formed on the mercury surface. It is necessary in al] these 

 investigations that the current pass in such a way as to produce 

 hydrogen polarization. Oxygen polarization does not change 

 the capillary constant in any regular manner. Moreover the 

 use of even coui}3aratively low potentials with oxygen polariza- 

 tion causes chemical action so that the surface of the mercury 

 is fouled, which seriously interferes with the action of the in- 

 strument. Two series of deflections of the meniscus for given 

 electro-motive forces were made. In one series hydrogen po- 

 larization was used, and in the other, oxygen. The series with 

 hydrogen polarization, was regular, as before ; but that of oxy- 

 gen, except for very low potentials, was so irregular that no 

 conclusions could be drawn. Apparently the position of the 

 meniscus depended largely on the chemical action caused by 

 the current. 



In using Lippmann's electrometer it might be convenient to 

 dispense with the mercury contact and employ only sulphuric 

 acid and platinum. A set of observations was accordingly 

 taken, in which the mercury was removed from the vessel Gr- 

 and the platinum wire served as electrode. The surface of the 

 platinum in contact with the sulphuric acid being so small, it 

 was found that, owing to polarization of the platinum-sulphuric 

 acid surface, a given potential would produce a deflection only 

 a fraction of that produced with mercury as electrode. With 

 the platinum electrode the maximum point on the curve was 

 for potential about l - 5 Dan. instead of - 9 Dan. as was the case 

 with the mercury electrode. The small capacity of the plat- 

 inum electrode will sufficiently account for this. If, then, plat- 

 inum electrodes be used in place of mercury, care should be 

 taken to have their capacity great compared with the capacity 

 of the mercury meniscus. 



It should be noted, then, that, in actual use, Lippmann's 

 electrometer makes a good means of measuring low potentials, 

 up to 0'6 or 0*7 Dan. ; that care must be taken to avoid oxygen 

 polarization and employ hydrogen polarization only ; that the 

 inner surface of the glass tube should be kept free from dust 

 and traces of acid, which occasion irregularities in the deflec- 

 tion of the mercury. The instrument in this form can be used 

 to advantage for comparing E. M. F. of different batteries. 

 Only a part (a known fraction) of the current should be used ; 

 as, otherwise, the limits of the instrument might be exceeded. 

 By using a micrometer for reading deflections, about YooTo 

 Daniell's cell E. M. F. can be detected. The electrometer 

 might be used successfully in many cases for a galvanometer, 

 especially where it is desirable not to alter the main current by 

 a derived galvanometer circuit. It may be recommended for 



Am. Jour. Sci.— Third Series, Vol. XXXV, No. 206.— Feb., 1888. 

 9 



