68 J. Whitmore — Method of Increasing 



others. In the instrument which was used, this tube had a 

 diameter of about a centimeter, and its extremity was curved 

 so that the terminal portion was at right angles to the vertical 

 portion. Platinum wires were used as electrodes in the usual 

 manner. 



As Lippmann has noted, the electrometer acquires a charge 

 during the operation of filling, and therefore the instrument 

 should be short-circuited for an hour or more before using so 

 that the charge may be wholly dissipated, and the mercury col- 

 umns stand at their true zero position. After the instrument 

 had been filled in the manner above described, it was firmly 

 mounted and placed on a pier before the cathetometer. Then 

 the curve showing the relation between the deflections was 

 constructed. Thus the deflections of the mercury column for 

 electromotive forces varying from zero to that of three Daniell 

 cells were observed on the cathetometer scale in hundredths of 

 a millimeter, the readings being taken for electromotive forces 

 differing from each other by - l of that of a Daniell cell. 



From this curve the absolute value of the electromotive 

 force corresponding to any deflection can be determined by 

 taking the deflection produced by a standard Clark cell as a 

 means of comparison. For example, it was found that the 

 electromotive force of the Clark standard was 1*360 times 

 that of a Daniell cell, and hence the electromotive force of 

 the Daniell cell was 1*051 volts. Accordingly, when the 

 curve, or empirical graduation of the instrument has been 

 made, the voltage of other cells can be obtained immediately 

 and with great facility. The curve gives directly the value of 

 the cell in terms of the Daniell cell, and hence as shown above, 

 the absolute value of the Daniell cell having been determined, 

 that of the cell which was to be measured, is calculated by 

 simple proportion. 



It is noteworthy, that not only is this electrometer valuable 

 as a means of quickly comparing low electromotive forces, but 

 that a high degree of accuracy is attainable in this comparison. 

 Thus a number of cells, which had been standing in the labo- 

 ratory were measured with the capillary electrometer and 

 immediately afterward the absolute values found by the com- 

 pensation method. 9 

 This method of meas- 

 urement is shown in the 



annexed diagram, fig. 2. Balance,, I'l'l'l 1 - 1 1 'I ' J'J 1 



Thomson's electric bal- i— -J 



ance gave the value of L [r Hr \^ /^\ % T ~ 



the strength of current. ' — LH-^\5 ' ' — ^~^ 



The following table 



exhibits the results of G, Galvanometer. R, R, Resistance Boxes. 



the measurements. c - Cel1 tested - K - Ke ^- 



3 



