64 J. Whitmore — Method of Increasing 



Art. X. — A Method of Increasing the Range of the Capil- 

 lary Electrometer • by John Whitmore. 



[Contributions from the Sloane Physical Laboratory of Tale College.] 



The value of the capillary electrometer as an instrument of 

 scientific research is now generally recognized, and its applica- 

 tions are becoming more numerous, both as an extremely deli- 

 cate test for small electromotive forces and also, as recently 

 pointed out by Burch,* as a means of studying variations of 

 differences of potential. In view of these facts it is desirable 

 that the range of this instrument be increased. 



The characteristic curve of Lippmann's electrometer is 

 slightly different from a straight line for electromotive forces 

 from # 00 to 045 volt, and consequently the instrument is 

 most applicable to the measurement of potential-differences 

 not exceeding O50 volt, although the direct measurements may 

 be carried to about 1*00 volt, since 1/20 volts are required to 

 produce continuous electrolysis in the instrument. The follow- 

 ing experiments were undertaken in order to determine, if the 

 range of the electrometer could be increased by arranging cells 

 in series. 



If a capillary tube is filled with alternate globules of mercury 

 and' dilute sulphuric acid, the difference of potential which can 

 be maintained between the extremities of the series increases 

 proportionally with the number of globules of mercury. Thus 

 with three globules, this difference of potential is about three 

 volts. If the terminals of the apparatus are connected with 

 the poles of a battery, the globules immediately begin to move 

 along the tube, and the motion continues until the opposing 

 force produced by the polarization of the globules of mercury 

 causes the electric current to cease. The extent of the move- 

 ment of the globules increases with the electromotive force. 



Since the end of each of the globules nearer the negative 

 pole of the system, receives oxygen polarization and the other 

 hydrogen polarization, one surface of the mercury is speedily 

 oxidized. Such an oxidation causes the movement of the 

 globules to become irregular, and prevents an accurate measure- 

 ment of electromotive force. In Lippmann's electrometer, the 

 surface of the mercury, which receives oxygen polarization, is 

 about ten thousand times greater than that upon which hydro- 

 gen is accumulated, and by this means the surface density of 

 the oxygen is diminished in the same ratio, and hence also the 

 oxidation is decreased. 



In order, therefore, to obtain a similar relation between the 



*Proc. Roy. Soc. of London, vol. xlviii, p. 89. 



