xlviii INTRODUCTION 



nate current methods. Probably the most accurate value is that given by Birge 

 (seep. 77). 



i international ohm= i. 00051 ±0.00002 absolute ohms, 



or, in other words, while one international ohm is represented by a mercury 

 column 106.300 cm long as specified above, one absolute ohm requires a similar 

 column 106.246 cm long. 



CURRENT 



The Silver Voltameter. — The silver voltameter is a concrete means of meas- 

 uring current in accordance with the definition of the international ampere. As 

 used for the realization of the international ampere "it consists of a platinum 

 cathode in the form of a cup holding the silver nitrate solution, a silver anode 

 partly or wholly immersed in the solution, and some means to prevent anode 

 slime and particles of silver mechanically detached from the anode from reach- 

 ing the cathode. As a standard representing the international ampere, the 

 silver voltameter includes also the chronometer used to measure time. The 

 degree of purity and the mode of preparation of the various parts of the vol- 

 tameter affect the mass of the deposit. There are numerous sources of error, and 

 the suitability of the silver voltameter as a primary standard of current has 

 been under investigation since 1893. Differences of as much as 0.1 per cent or 

 more may be obtained by different procedures, the larger differences being 

 mainly due to impurities produced in the electrolyte (by filter paper, for instance). 

 Hence, in order that the definition of current be precise, it must be accompanied 

 by specifications for using the voltameter." 



The original specifications were recognized to be inadequate and an inter- 

 national committee on electrical units and standards was appointed to com- 

 plete the specifications. It was also recognized that in practice standard cells 

 would replace secondary current standards so that a value must be fixed for the 

 electromotive force of the Weston normal cell. This was attempted in 1910 at 

 the Bureau of Standards by representatives of that institution together with one 

 delegate each from the Physikalische-Technische Reichsanstalt, The National 

 Physical Laboratory and the Laboratoire Central d'Electricite. Voltameters 

 from all four institutions were put in series under a variety of experimental con- 

 ditions. Standard Weston cells and resistance standards of the four laboratories 

 were also intercompared. From the joint comparison of standard cells and 

 silver voltameters particular values were assigned to the standard cells from 

 each laboratory. The different countries thus have a common basis of measure- 

 ment maintained by the aid of standard cells and resistance standards derived 

 from the international voltameter investigation of 19 10. 



It was not found possible to draw up satisfactory and final specifications for 

 the silver voltameter. Provisional specifications were submitted by the U. S. 

 Bureau of Standards and more complete specifications have been proposed in 

 correspondence between the national laboratories and members of the inter- 



