ROSA AND WOLFF: INTERNATIONAL ELECTRICAL UNITS 263 



From the point of view of the methods of preparation of the 

 mercurous sulphate, the following differences exist among the 

 cells : 



At the PTR, by precipitation by adding hot sulphuric acid to 

 acid mercurous nitrate, or the reverse. 



At the NPL, by precipitation by adding mercurous nitrate 

 to sulphuric acid. 



At the NBS, by electrolysis with direct current. 



At the LCE, part by electrolysis with direct current, the rest 

 by alternating current electrolysis. 



All the cells were compared several times during the work of 

 the delegates. As a result of these comparisons, the technical 

 committee decided to choose as " value of the Weston Normal 

 Cell, the mean value of the cells submitted by the delegates of 

 the four laboratories. This mean has been calculated in the 

 following manner: The mean of the values of the standard cells, 

 presented by each delegate, has first been taken, and then the 

 mean of the four numbers thus found." 



Differences in Microvolts between Separate Group Means and the Bureau 



of Standards Basis of Reference 



The cell (no. 109) used in making the voltameter measure- 

 ments, was frequently compared with the original reference cells 



