|-KI!IU ARY 3. 1905.] 



SCIENCE. 



of length, mass and time, and these having 

 been selected (namely, the centimeter, 

 iivnm and si^ecmd), the definitions or speci- 

 fications of the electrical units follow log- 

 ically, but their concrete expression in 

 actual standards that can be employed in 

 electrical measurements can only be at- 

 tained after most painstaking researches 

 in what are called absolute measurements. 

 The two of these three units which have 

 been so determined are the ohm and the 

 ampere. As all other electrical units are 

 based upon these, it is of the greatest im- 

 portance that they be determined with the 

 utmost exactness. At the International 

 Electrical Congress at Chicago, in 1893, 

 they were redefined in accordance with the 

 results of the best determinations made up 

 to that time. The ohm is specified in terms 

 of the resistance of a column of mercury 

 106.3 em. long, having a cross-section of 

 one square millimeter; the ampere in terms 

 of the quantity of pure silver it will de- 

 posit electrolytically per second from a 

 solution; the volt in terms of the electro- 

 motive force of the standard Clark cell. 

 An immense amount of work has been done 

 by numerous investigators in various coun- 

 tries of the world in the detei-mination of 

 the values of these electrical units, and the 

 figures adopted in the definitions un- 

 doubtedly come very near the truth. 

 Nevertheless, we know from subsequent 

 work that at least two of these units are 

 very slightly in error, and one of the most 

 important problems before the bureau of 

 standards is the redetermination of these 

 fundamental units. The error in ques- 

 tion is small, so small as to be of no conse- 

 quence in engineering and commercial 

 work. But scientifically it is important, 

 and as instruments and methods are im- 

 proved year by year, any small discrep- 

 ancies in our fundamental i;nits become 

 of more and more significance. The Na- 

 tional Physical Laboratory of England, 



the Pliysikalisch-Technische Reichsanstalt 

 of Germany and the National Bureau of 

 Standards, as well as a few private investi- 

 gators in this country and abroad, are all 

 working in the same direction. The recent 

 International Electrical Congress at St. 

 Louis provided for the formation of an in- 

 ternational commission, whose function it 

 shall be to foster and in some degree direct 

 and coordinate researches of this character. 

 This commission will probably organize and 

 enter upon its work during the coming 

 year. The difficulties to be overcome are 

 so great that only the most elaborate re- 

 seai'ches carried out under the most favor- 

 able circumstances can be expected to 

 bring us appreciably nearer the desired 

 goal. Two researches at the bureau of 

 standards during the past year gave re- 

 sults of value preparatory to the rede- 

 termination of the ampere in absolute 

 measure. OiTe was by Dr. Wolff, showing 

 how to overcome one of the defects of the 

 standard cell ; a new method of preparing 

 the mercurous sulphate yielding a crj^stal- 

 line product which gives cells of more uni- 

 form electromotive force than formerly. 

 Professor Carhart, of Ann Arbor, who has 

 been engaged upon this subject for some 

 time, arrived independently at the same 

 result even earlier, the results being an- 

 nounced by both men at the same meeting 

 in Washington in April last. The other 

 investigation was by Dr. Guthe, who, after 

 carefully studying all the various forms of 

 silver voltameters which have been pro- 

 posed, showed that although different 

 kinds gave slightly different results, cer- 

 tain ones when properly handled, gave 

 practically identical results, and hence 

 could be depended upon for measuring 

 current to a very high order of accuracy. 

 Dr. Wolff is continuing his work on stand- 

 ard cells and Dr. Guthe is now engaged in 

 the absolute measurement of current, by 

 means of a new electrodynamometer. 



