TABLE 261 . 



251 



ABSOLUTE MEASUREMENTS OF CURRENT AND OF THE ELECTROMO- 

 TIVE FORCE OF STANDARD CELLS. 



The most probable value of the Weston cell at 20 is 1.0182 volts, assuming the International 

 ohm to be io 9 c. g. s. units and the volt to be io 8 c. g. s. units. The corresponding value of the 

 Clark cell, as prepared at present, at 15, is 1.4324 volts. 



The legal values of the Weston cell, however, are different in different countries, as follows : 



United States (Bureau of Standards) 1.019125* v. at 20 



Germany (Physikalisch-Technische Reichsanstalt) 1.0186 volts at 20 



England (National Physical Laboratory) 1.0184 volts at 20 



The value of the Weston standard cell, used in the United States, is based upon the value 

 adopted by the Chicago Electrical Congress (1893) for the Clark cell. The value used by Ger- 

 many was adopted in 1896, and is based on Kahle's work at the Reichsanstalt. The value used 

 in England was adopted January I, 1909, and is based on the recommendation of the London 

 Electrical Conference of 1908. It is expected that a new value will soon be agreed upon by the 

 International Committee on Electrical Units and Standards, which will be adopted generally in all 

 countries. 



The value of the electrochemical equivalent of silver is different when filter paper (Rayleigh 

 form), silk, or other textile is used to separate the anode from the cathode from what it is when a 

 porous cup is employed. The value found is also affected by the addition of silver oxide to the 

 silver nitrate solution. The legal value in all countries is i.i 18 mg. of silver per coulomb, and this 

 is nearly the value found when using a porous cup voltameter, and the best determinations of the 

 current that have been made by absolute current balances. Some corrections have been made to 

 the figures given in the above table for the excess due to filter paper, but such corrections are very 

 uncertain. 



* Based on 1.0x89 at 25 C. 

 SMITHSONIAN TABLES. 



