264 EOSA AND WOLFF: INTERNATIONAL ELECTRICAL UNITS 



of the Bureau of . Standards and its value in terms of the mean 

 of the group means was thus known. The cells set up by members 

 of the Committee in Washington confirmed in the main the con- 

 clusions derived from the cells brought from the home labora- 

 tories, although considerably larger differences than shown in 

 the above table were found. 



3. DESCRIPTION OF THE EXPERIMENTS ON THE VOLTAMETER 



The current in the voltameter circuit was held at such a value 

 that the drop of potential over the 2-ohm resistance standard 

 was equal to the electromotive force of the standard cell used. 

 To correct, however, for errors in the potentiometer, such as 

 errors in the adjustment of the relative values of the coils, thermo- 

 electromotive forces, changes in the potentiometer current and 

 the like, a control circuit was arranged. Cell no. 109 whose 

 value was to be determined by means of the voltameters, was 

 joined in series with a sensitive galvanometer, so that by depress- 

 ing a key the drop of potential over the standard resistance could 

 be opposed directly to that of the standard cell. This circuit 

 served as the ultimate control for the current. The procedure 

 adopted was as follows: While one observer adjusted the current 

 until the electromotive forces in the circuit just described were 

 balanced, a second observer at the potentiometer took note of 

 the resting place of the potentiometer galvanometer which corre- 

 sponded to this balance. The reading of the potentiometer 

 galvanometer was then held at this point, by means of the Kelvin 

 rheostat, with an occasional check on the correct resting point 

 by the method just described. This procedure not only allowed 

 the errors of the potentiometer to be eliminated, but permitted 

 a second observer to judge of the steadiness of the current at 

 the same time. 



The duration of each experiment was timed by a chronograph 

 recording the ticks of a standard Riefler clock. 



The deposits were weighed in a constant temperature room 

 constructed especially for the purpose and a high order of accu- 

 racy was obtained. 



