Makch 16, 1900.] 



SCmNGE. 



409 



cells on hand, well within i^-J^j^ . The mean 

 electro-motive force of the three dozen or 

 more cells furnishes thei-efore a standard of 

 reference which may be relied on within 

 this limit. In addition to the Clark cells, 

 a number of Cadmium cells, having consid- 

 erable advantages over Clark cells, have 

 been set up, but owing to lack of time no 

 comparisons have as yet been made. New 

 cells of both types are to be added from 

 time to time, and intercompai-ed with the 

 old ones to determine whether any observ- 

 able changes have taken place. 



The relation which the E. M. F. of the 

 Clark cell bears to that of the Cadmium cell 

 will also be periodically determined, furnish- 

 ing an additional check on the constancy of 

 the standards of both types. 



With an accurately calibrated potentio- 

 meter, and reliable standards of electro- 

 motive force the office is thus prepared to 

 undertake the verification of direct current 

 volt-meters and millivolt-meters. 



MEASUREMENT OF ELECTEIC CTJEEENTS. 



A grave mistake was made by the Inter- 

 national Congress in concretely defining all 

 three of the principal electrical units, the 

 ohm, volt and ampere, which are neces- 



E 

 sarily connected by the fixed relation C = p- 



Hence only two of the definitions are inde- 

 pendent, the chances being infinitesimal 

 that the three definitions satisfy the relation 

 between them on account of the relatively 

 large errors in the absolute determinations 

 on which they are based. Indeed, it al- 

 ready seems that the volt, as defined in 

 terms of the Clark cell is in error by almost 

 .1%. 



Since the standards of resistance and 

 electromotive force, as specified by the In- 

 ternational Congress, are certainlj' repro- 

 ducible within Y^%, the unit of current 

 intensity, defined as the current which will 

 flow through a conductor of unit resistance. 



there being unit difference of potential be- 

 tween its terminals, would be fixed within 

 the same limits of error. 



Instead, it is defined in terms of the 

 electro-chemical equivalent of silver. The 

 voltametric measurement of a current i& 

 limited by the size of the apparatus avail- 

 able and is besides impossible under ordi- 

 nary circumstances with the above accuracy. 



Lord Eayleigh is of the opinion that his 

 determination of the electro-chemical equiv- 

 alent of silver may be in error by as much 



asxV%- 



Retaining for the present the legal defi- 

 nitions of the ohm and volt, currents maj?^ 

 be consistently measured by the fall in po- 

 tential or potentiometer method, in terms 

 of the specified standards of reference. 



It is proposed to base all direct current 

 measurements on these principles. With 

 suitable low resistance standards for the 

 measurement of heavy currents and with 

 the set of Clark cells already on hand, the 

 Office is prepared to undertake the verifi- 

 cation of direct current ammeters within 

 the limits practically set by the current 

 generating apparatus on hand. 



To summarize, the Office is therefore 

 practically equipped for the verification of 

 following classes of apparatus, viz : 



Resistance Standards. — Coil of the follow- 

 ing denominations: 1, 2, 5, 10, 100, 1000, 

 10,000 ohms. 



Low Resistance Standards for current meas- 

 urements of the following denominations : 

 0.1, 0.01, 0.001 and 0.0001 ohms. 



Resistance boxes. 



Potentiometers. 



Ratio coils. 



Standards of Electro-motive Force. — Clark 

 standard cells and other standard cells. 



Direct Current Measuririg Apparatus. Milli- 

 voltmeters and voltmeters up to 150 volts. 



Ammeters up to 50 amperes. 



For the verification of direct current 

 measuring apparatus of even moderate 



