30 REroRT— 1884. 



of the comparison made by the Secretary and Mr. H. M. Elder, with a 

 table of the values arrived at, is given in Appendix I. Further ex- 

 periments on the temperature coeflBcients of these coils are in progress. 

 During the year, twelve coils have been compared with the B. A. 

 standards, and certificates of their values issued by the Secretary. A 

 Table of the values found is given in Appendix II. 



At the Southport meeting of the Association a grant was made to 

 defray the expense of procuring standards of resistance in terms of the 

 ohm. At a meeting of the Committee held in March, 1884, it was 

 decided to defer the purchase of these till after the meeting of the Paris 

 Congress, and a resolution was passed to the effect that ' In the event of 

 the Paris Congress adopting any definite standard of resistance, 

 standards be ordered for the Committee in accordance with that value.' 



The Paris Congress adopted as a standard, to be called the ' legal 

 ohm,' the resistance at 0°C. of a column of a mercury 106 centimetres 

 long, and one square millimetre in section. The standard resistances at 

 present in use being B.A. units, it became necessary to assume a relatiL'n 

 between the B.A. tinit and the legal ohm, in order to construct coils whose 

 resistance should be one legal ohm. This relation has been determined 

 by various observers with slightly different results, and a meeting of the 

 Committee was held on June 28 to consider the question. At this meet- 

 ing the following resolution, proposed by Professor W. G. Adams, 

 seconded by Lord Rayleigh, was carried : — ' That, for the purpose of | 



issuing practical standards of electrical resistance, the number of B.A. fl 



units adopted as the resistance of a column of mercury 100 cm. in length, 

 1 sq. mm. in section, at 0°C., be "9540. 



Taking this number, then 



1 legal ohm=l-0112 B.A. units. , 

 1 B.A. unit=-9889 legal ohms. 



Coils having respectively a resistance 1, 10, 100, 1,000 and 10,000 

 legal ohms have been ordered, two of each value, so that, by frequent 

 comparison of one with the other, an accident to either may be checked. 

 These standards are to have their correct values at temperatures near 

 1-5° C. 



The two 1-ohm coils have been sent by the makers, and their testing 

 is being proceeded with. When this is complete the Committee will be 

 in a position to test and certify to the values of coils in terms of the 

 legal ohm. 



They propose that the certificate shonld run as follows : — 



* This is to certify that the resistance coil X li^s been tested by the 

 Electrical Standard Committee, and that its value at a temperature of 

 4° centigrade is P legal ohms. 



' It has been assumed, for the purposes of this comparison, that 1 legal 

 ohm is equal to 1-0112 B.A. units.' 



The coils will be stamped with the monogram ^ and a reference 



number. 



A portion of the grant has been expended in some additions to the 

 ■wire bridge belonging to the Committee, which have added greatly to its 

 utility, while two thermometers for the testing room have been purchased. 



The Committee would ask, in conclusion, that they may be reappointed, 

 •with the addition of the name of Mr. W. N. Shaw, in order to continue 

 the work of issuing standards of resistance. 



