19 



TABLE 4.— THE NEW (1948) SYSTEM OF ELECTRICAL UNITS 12 



In pursuance of a decision of the International Committee on Weights and 

 Measures, the National Bureau of Standards introduced, as of January 1, 

 1948, revised values of the units of electricity. This consummated a movement, 

 initiated in 1927 by the American Institute of Electrical Engineers, asking that 

 the National Bureau of Standards undertake the additional research necessary 

 in order that the absolute ohm and absolute ampere based on the cgs electro- 

 magnetic system and the absolute volt, watt, and other units derived from 

 them could be legalized in place of the international ohm and ampere and their 

 derived units. This work was done, and the magnitude of the old international 

 units in terms of the adopted absolute units is given in Table 5. This means 

 that the electrical units now in use represent, as nearly as it is possible to make 

 them, exact multiples of the cgs emu system, with the numerical relations 

 shown in Table 6. Units of the new system will actually be maintained, as 

 were the old international units, by groups of standard resistors and of standard 

 cells, and consequently the change to be made is most simply represented by 

 stating the relative magnitudes of the ohms and of the volts of the two systems. 



During the period of transition to the new units, in order to avoid any doubt 

 as to the units used in giving precise data, the International Committee on 

 Weights and Measures recommended that the abbreviations int. and abs. be 

 used with the names of the electrical units. In a few years this will be un- 

 necessary, except when referring to old data. 



The international units were intended to be exact multiples of the units of 

 the centimeter-gram-second electromagnetic system, but to facilitate their re- 

 production, the ampere, the ohm, and the volt were defined by reference to 

 three physical standards, namely (1) the silver voltameter, (2) a specified 

 column of mercury, and (3) the Clark standard cell. This procedure was 

 recommended by the International Electrical Congress of 1893 in Chicago and 

 was incorporated in an Act of Congress of July 12, 1894. However, modifica- 

 tions of the international system were found to be necessary or expedient for 

 several reasons. The original proposals were not sufficiently specific to give 

 the precision of values that soon came to be required, and the independent defi- 

 nitions of three units brought the system into conflict with the customary 

 simple form of Ohm's Law, l — E/R. Furthermore, with the establishment 

 of national standardizing laboratories in several of the larger countries, other 

 laboratories no longer needed to set up their own primary standards, and 

 facility of reproduction of those standards became less important than the 

 reliability of the units. 



In preparation for the expected change in units, laboratories in several 

 countries made absolute measurements of resistance and of current. The re- 

 sults of these measurements and the magnitudes of the international units as 

 maintained in the national laboratories of France, Great Britain, Germany, 

 Japan, the U.S.S.R., and the United States were correlated by periodic com- 

 parisons of standard resistors and of standard cells sent to the International 

 Bureau of Weights and Measures. Nearly all the absolute measurements at 

 the National Bureau of Standards were carried out under the direct supervision 

 of Harvey L. Curtis, and the results of such measurements at the Bureau 

 accepted by the International Committee on Weights and Measures at its 

 meeting in Paris in October 1946 are as follows : 



1 mean international ohm = 1.00049 absolute ohms 

 1 mean international volt = 1.00034 absolute volts 



12 Nat. Bur. Standards Circ. C-459, 1947. 



SMITHSONIAN PHYSICAL TABLES 



