March 16, 1900.] 



science: 



403 



THE FACILITIES AFFOBDED BY THE OFFICE 

 OF STANDARD WEIGHTS AND MEAS- 

 URES FOR THE VERIFICATION OF 

 ELECTRICAL STANDARDS AND 

 ELECTRICAL MEASURING 

 APPARATUS. 

 The need of adequate facilities for the 

 official verificatioa of standards and meas- 

 uring apparatus of all kinds, has long been 

 recognized by American physicists. The 

 Office of "Weights and Measures, with its 

 small force, modest equipment and insuf- 

 ficient appropriations, has endeavored to 

 meet any demands imposed upon it within 

 the limits thus set. 



The object of this article is to describe 

 what has thus far been accomplished along 

 electrical lines, and in this connection a 

 brief history of the units of reference may 

 not be considered out of place. 



The origin of standards of quantity and 

 value dates back as far as the earliest his- 

 torical records. The transition from the 

 crude measures of antiquity to the systems 

 meeting present requirements has, however, 

 been exceedingly slow, although the growth 

 of commercial intercourse gradually ledj[^to 

 the introduction of more precise standards. 

 The requirements of accuracy were, nev- 

 ertheless, quite modest until physics began 

 to emerge from its qualitative stage to as- 

 sume the dignity of an exact science. 



The number, and complexity of physical 

 quantities at first led to the adoption of 

 more or less arbitrary standards of refer- 

 ence, often based on the physical properties 

 of some definite substance, e. g., the calorie, 

 and as long as the relations between phys- 

 ical quantities remained obscure, such rela- 

 tive standards sufficed. With the develop- 

 ment of the science came the recognition of 

 the desirability of a consistent system of 

 units, which was strongly brought to the 

 front, when it became necessary to measure 

 magnetic quantities. 



The happy solution of this problem sug- 



gested by Gauss and extended to the meas- 

 urement of electrical and electro-magnetic 

 quantities by "Weber, constituted a great 

 advance in the science of Metrology, the 

 units being defined in terms of the mechan- 

 ical actions to which they give rise. Since 

 all mechanical units can be expressed in 

 terms of three independent fundamental 

 units, such as those of length, mass and 

 time, the magnetic and electrical units of 

 Gauss and "Weber are thus expressible in 

 terms of the same fundamental units and 

 are therefore entirely independent of the 

 physical properties of arbitrarily chosen 

 substances. Hence they are called absolute 

 units, their magnitudes depending solely on 

 the choice of the fundamental units. 



The units of length and mass selected 

 were usually multiples or sub-multiples of 

 the corresponding metric units, the metric 

 system being the only one meeting the 

 highest scientific and practical requirements 

 and being already in extensive use. "While 

 at first uniformity in the selection of the 

 magnitudes of the fundamental units was 

 lacking, the centimeter, gram and second 

 have now been universally adopted, the de- 

 rived units being known as C. G. S. units. 

 Practical measurements of electrical re- 

 sistance were, however, first referred to a 

 variety of arbitrary standards, e. g., the re- 

 sistance of a given length of copper or iron 

 wire of given diameter, and so long as 

 electrical measurements were confined to 

 the laboratory, the length referred to was 

 usually a few feet, but with the practical 

 applications of electricity to telegraphy and 

 submarine signaling, this length became 

 inconveniently small and was therefore re- 

 placed by fathoms, miles, etc. Fortunately 

 none of these units ever gained general ac- 

 ceptance. In 1848 Jacobi pointed out that 

 it would be far preferable to adopt, as a 

 universal standard, the resistance of a cer- 

 tain piece of wire, copies having the same re- 

 sistance being easily constructed. 



