406 



SCIENCE. 



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



B. A. units, indicating a change in the coils 

 themselves, or in their assumed relation to 

 the Siemens unit or in both. 



Matters have been still further compli- 

 cated in England by the legalization of the 

 resistance of a coil marked ' Board of Trade 

 Standard verified 1894,' and adjusted with 

 reference to the Cambridge Standards, as the 

 unit of resistance. 



A still more radical suggestion was made 

 in that country several years ago, by Profes- 

 sor A. Viriamu Jones and Professor W. 

 E. Ayrton, that each government adopt a 

 Lorenz apparatus from which to derive by 

 absolute measurements the unit of resist- 

 ance. The practical substitution of an ab- 

 solute method with its disadvantages as 

 pointed out above, for a purely relative one 

 seems to be taking a step or indeed several 

 steps backward, but maj' be taken as indi- 

 cating the lack of confidence in the perma- 

 nency of the B. A. coils. 



After the legalization of the International 

 Electrical Units it became the imperative 

 duty of our government to provide facili- 

 ties for the official verification of electrical 

 standards and electrical measuring appa- 

 ratus, especially in view of the continually 

 increasing importance of the applications of 

 electrical energy to the industral arts. This 

 function obviously devolved upou the OfiBce 

 of Standard Weights and Measures, already 

 equipped for the verification of standards 

 of length, mass, capacity, etc. 



Owing, however, to the limited force and 

 to tlie still more limited appropriations 

 available for the purchase of apparatus, 

 practically nothing could be done until 

 July 1, 1897, when the appointment of a 

 verifier was authorized, but unfortunately 

 progress has frequently been interrupted for 

 long periods by thepressureof routine work. 

 It was determined from the outset that 

 it should be the aim of the Office to provide 

 facilities for measurements to any degree of 

 accuracj' likely to be set even by the most 



exacting demands of modern science. The 

 first steps taken by the Office consisted in 

 working out a general plan and providing 

 the most needed apparatus and facilities. 



STANDARDS OF RESISTANCE. 



To avoid the delay which would natur- 

 ally arise from the construction of primary 

 mercury standards, it was decided to refer 

 all measurements of electrical resistance to 

 the mean value of a number of wire coils, 

 known in terms of the best existing mer- 

 cury standards. The general excellence of 

 coils of the Reichsanstalt type, the extremely 

 small temperature coefficients and thermo- 

 electromotive power with respect to copper 

 of manganin and the permanency of coils 

 of that material as shown by long con- 

 tinued observations at the Reichsanstalt, 

 decided in favor of standards of the above 

 description. 



Four unit coils were purchased and were 

 standardized at the Reichsanstalt where 

 they were kept under observation for about 

 one month, during which period they de- 

 creased in value by about .0015%. This, 

 according to the maker, is the normal be- 

 havior of the material which undergoes, 

 after the artificial aging, a small decrease 

 in resistance followed by a slow increase. 

 In addition to these coils the Office pos- 

 sesses two other coils of the same type pur- 

 chased several years ago. Periodic inter- 

 comparisons between the old coils and the 

 new ones will be made so that any relative 

 changes will be made manifest, the two 

 hardly having the same rate of change. 

 Two new manganin coils have been ordered 

 and are also to be referred to the German 

 Standards. On their receipt, they will fur- 

 nish us positive evidence in regard to any 

 changes which may have taken place in the 

 preliminarj' standards adopted. The ap- 

 proximate corrections at any time can then 

 be determined by simple interpolation. 

 To fix this standard, the construction of 



