March 16, 1900.] 



SCIENCE. 



405 



assumed value by more than one per cent, 

 and soon after, this difference was substanti- 

 ated by a number of other investigators. 



In 18S1 a call was issued, in connection 

 with the first Paris electrical exhibition, for 

 an international electrical congress for the 

 purpose of adopting definitions of the elec- 

 trical units to serve as a basis for legislative 

 enactments. Numerous mercury standards 

 had in the meantime been constructed and 

 had been found to agree most satisfactorily 

 with one another ; moreover, the results of 

 a considerable proportion of the absolute 

 determinations made had been referred, di- 

 rectly or indirectly, to the Siemens unit. 

 In view of this, the Paris Congress passed 

 a resolution recommending that all absolute 

 determinations in the future be expressed 

 as the resistance of a column of mercury 

 of stated length, 1 sq. mm. in cross section 

 at the temperature of melting ice, and 

 that the C. G. S. electro-magnetic system 

 of units be adopted. The desirability of 

 making new determinations of the ohm was 

 urged. 



Three years later the Congress reassem- 

 bled at Paris and adopted 106 cm. as the 

 length of the specified column of mercury, 

 individual results still diflfering by as much 

 as ± .5 per cent. The unit of resistance 

 thus defined was called the legal ohm, but 

 while it was never legalized anywhere, it 

 nevertheless came into extensive use, es- 

 pecially in England and in America. 



Absolute methods were gradually im- 

 proved, sources of error were pointed out, 

 and eliminated as far as possible and ac- 

 cordingly the results of absolute determi- 

 nations, made by the most radically differ- 

 ent methods, became more concordant. 



The International Electrical Congress 

 (which met in Chicago in 1893), through 

 its Chamber of Delegates, ofBcially repre- 

 senting all the leading governments, there- 

 fore adopted, as the unit of i-esistance, the 

 mean of all the best determinations, the re- 



sistance at 0° C. of a column of mercury 

 106.3 cm. in length and of uniform cross 

 section having a mass of 14.4521 gm. (equiv- 

 alent to a cross section of 1 sq. mm., the 

 density of mercury being assumed to be 

 13.5956). The unit of resistance and the 

 other electrical units defined by the In- 

 ternational Congress were legalized in the 

 United States by Act of Congress in 1894, 

 and have also been legalized in the other 

 countries represented. 



It was generally supposed that the vari- 

 ous governments would sooner or later take 

 up the construction of mercury standards 

 as called for in the definition since each had 

 already been provided by the International 

 Bureau of Weights and Measures with the 

 fundamental standards of mass and length. 



The Imperial Physico-Technical Eeichs- 

 anstalt in Berlin has already begun this 

 task with its characteristic thoroughness. 

 Two one ohm mercurial standards were 

 constructed and later, a third together with 

 a ^ and a two ohm tube. Widely different 

 cross sections were purposely selected to 

 avoid possible sources of error. The cali- 

 bration and intercomparison of these stand- 

 ards leaves almost nothing to be desired, 

 measurements made by two different meth- 

 ods, both yielding the same results within 

 the limits of experimental error. 



Twelve mercury copies were also con- 

 structed ; these, together with 7 working 

 standards of manganin wire, periodically 

 referred to the primary standards, complete 

 the list. 



In England, on the other hand, the B. 

 A. coils have still been retained as primary 

 standards. The legal ohm was defined in 

 that country by the relation, 1 Siemens unit 

 = .9540 B. A. units, and later the Inter- 

 national Ohm was defined by the relation, 

 1 Siemens unit = .95351 B. A. units. Ac- 

 cording to a comparison made several years 

 ago, 1 Siemens unit, as derived from the 

 Reichsanstalt Standards, is equal to .95341 



