ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 135 



countries, and two meetings of the Committee were held, at which dis- 

 cnssions took place. Major Cardew, of the Board of Trade, was present, 

 and took part in the discussion. Dr. von Helmholtz drew special 

 attention to the need for having a unit of resistance defined in terms of a 

 specified column of mercury, and pointed out that the difficulty arising 

 from the uncertainty of the relation between the centimetre and the 

 gramme might be avoided by defining the mass of mercury column of 

 given length, which has a resistance of cue ohm. After discussion the 

 following resolutions were agreed to : — 



1. That the resistance of a specified column of mercury be adopted 

 as the practical unit of resistance. 



2. That 14'4521 grammes of mercury in the form of a column of 

 uniform cross section 106'3 m. in length at 0" C. be the specified column. 



3. That standards in mercury or solid metal having the same resist- 

 ance as this column be made and deposited as standards of resistance 

 for industrial purposes. 



4. That such standards be periodically compared with, each other, and 

 also that their values be redetermined at intervals in terms of a freshly 

 set up mercury column. 



It was further agreed that these resolutions should be communicated 

 to the Electrical Standards Committee of the Board of Trade. 



With regard to the unit of current and of electromotive force, it was 

 agreed that the number '001118 should be adopted as the number of 

 grammes of silver deposited per second from a neutral solution of nitrate 

 of silver by a current of 1 ampere, and the value 1'484 as the electro- 

 motive force in volts of a Clark cell at 15° C. 



Dr. von Helmholtz expressed his full concurrence in these decisions, 

 which are, as he informed the Committee, in accord with the recommen- 

 dations which have already been laid by the Cnratorium of the Reichs- 

 anstalt, as well as by himself, before the German Government. 



The Committee wish to place on record their thanks to Dr. von 

 Helmholtz and the other visitors for the help they have aff'orded them in 

 coming to so satisfactory a conclusion. 



Dr. Lindeck laid before the Committee some information as to the 

 properties of the manganese alloy used at the Reichsanstalt for resistance 

 coils (see Appendix IV.), and it was agreed that it was desirable to obtain 

 copies of the German standards in manganese for further comparison with 

 the standards of the Association. The Committee therefore recommend 

 that they be reappointed, with the addition of the name of Mr. George 

 Forbes, and with a grant of 251., including an unexpended balance of 

 101. ; that Professor G. Carey Foster be Chairman, and Mr. R. T. 

 Glazebrook, Secretary. 



APPENDIX I. 



Information circulated by the Secretary for the Meeting of the Committee on 

 August 4, 1892, ivith additional Notes. 



The Report of the Electrical Standards Committee of the Board of 

 Trade is printed in the 'B.A. Report ' for 1891. Further information as 

 to the values of the units is given in the ' B.A. Report ' for 1890. The 

 following summary may be of use : — 



