404 



SCIENCE. 



[N. S. Vol. XI. No. 272. 



Jacobi carried this suggestion into prac- 

 tice by sending a piece of copper wire, since 

 known as ' Jacobi's Etalon ' to various 

 pliysicists for that purpose. 



In 1860 Werner von Siemens proposed 

 as a standard of resistance, the resistance, 

 at 0°C. , of a column of mercury, 1 sq. mm. 

 in cross section and 100 cm. in length. 



In 1861 a committee, composed of the 

 most eminent English physicists was ap- 

 pointed by the British Association to con- 

 sider the question of Standards of Electrical 

 Resistance. Correspondence was opened 

 with the leading foreign physicists and 

 various special investigations of the prob- 

 lems, with which the committee was con- 

 fronted, were undertaken by its members. 



The first question settled was that the 

 unit of resistance should be defined as a 

 multiple by an integral power of 10 of the 

 unit of Weber's absolute system, and that 

 the unit chosen should be of a convenient 

 magnitude. Accordingly, a unit equivalent 

 to 10^ C. G. S. units was adopted. 



This definition fixed the unit, but the 

 evaluation of a resistance in absolute meas- 

 ure requires the construction of especially 

 designed apparatus, having usually a limited 

 range of usefulness ; the determination of 

 instrumental constants, most frequently in- 

 volving tedious mathematical approxima- 

 tions, and in addition, the observations have 

 to be made with the greatest precision. On 

 the other hand, relative measurements re- 

 quire simpler apparatus and less skill in 

 manipulation, besides being, in most cases, 

 far more accurate than absolute measure- 

 ments. The construction of material 

 standards adjusted to the specified resist- 

 ance, determined once for all by a series 

 of absolute measurements, was therefore 

 decided upon. 



Investigations were made to determine 

 whether the absolute unit of resistance 

 could be accurately defined in terms of the 

 resistance of a definite portion of a definite 



substance. Pure metals in the solid and 

 liquid state and alloys were studied with 

 this end in view. On account of the ex- 

 cessive influence on the resistance produced 

 by small quantities of impurities in metals 

 and by small variations in the composition 

 of alloys and on account of the additional 

 difficulty of procuring chemically pure 

 materials, the choice was greatly limited. 

 Moreover solids had to be rejected on ac- 

 count of the marked effect of physical 

 changes produced by drawing, bending, an- 

 nealing, etc. 



Mercury, already recommended by Sie- 

 mens, was therefore the only material to be 

 further considered. Even this material was 

 rejected,' owing to large differences found to 

 exist between coils, supposedly adjusted to 

 agree with different German mercury stand- 

 ards, and the mercury standards constructed 

 by members of the committee. 



Having abandoned the above proposi- 

 tions, the alternative remained of construct- 

 ing material standards adjusted with refer- 

 ence to the absolute unit. 



A number of new alloys, in addition to 

 many already in use, were made and inves- 

 tigated. An alloy of 2 parts by weight of 

 silver to 1 part bj' weight of platinum was 

 finally selected as best meeting all require- 

 ments. A special form of resistance stand- 

 ard was also adopted. 



In 1863 and 1864 the values of certain 

 coils were determined in absolute units by 

 one of the methods proposed by Weber and 

 from these measurements the B. A. unit 

 was derived. A number of copies were 

 issued gratis by the Association and in ad- 

 dition arrangements were made to furnish 

 others at a moderate price. The B. A. 

 unit soon gained universal acceptance in 

 the English speaking countries, while the 

 Siemens unit still retained its supremacy 

 on the continent. 



In 1878 it was shown by Professor Row- 

 land that the B. A. unit differed from its 



