ON PKACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 105 



down exact directions in respect to the silver voltameter and the standard 

 cell. 



3. That if a proposal for a change in the accepted value of the ampere 

 is to be brought from any source before a formal congress to be held later, 

 an agreement in writing on the point should be come to previously 

 between the parties interested. If differences of opinion in the matter 

 cannot be removed, a new preliminary conference should be held. 



The same procedure should be observed in regard to the specification 

 for the silver voltameter and the standard cell, in the event of such 

 specifications being submitted to a formal conference from any quarter. 



The following iormal decisions were agreed to : — 



1. That only two electrical units shall be chosen as fundamental units. 



2. The international ohm, defined by the resistance of a column of 

 mercury, and the international ampere, defined by the deposition of silver, 

 are to be taken as the fundamental electrical units. 



3. The international volt is that electromotive force which produces 

 an electric current of one international ampere in a conductor whose 

 resistance is one international ohm. 



4. The Weston Cadmium Cell shall be adopted as the standard cell. 



Recommendations were also made as to realising the ohm, and some 

 particulars as to the Cadmium cell were agreed upon. 



These results were laid before the Board of Trade, and a Departmental 

 Committee, of which the Secretary was a member, drew up a report 

 reconmiending that an official conference should be invited to meet in 

 London, and it is understood that negotiations are now on foot with view 

 to summoning such a conference. 



During the year the work in connection with the absolute ampere balance 

 has been in progress, and is practically complete. Under the supervision of 

 Professor Ayrton and Mr. Mather a large number of determinations have 

 been made, and are most satisfactory. Detailed particulars are reserved 

 until the work is complete ; but there is little doubt that the balance is a 

 most excellent absolute instrument, and that the probable error of a 

 determination of current by means of it is only a few parts in 100,000. 



The investigation of the silver voltameter has been extended beyond 

 the limits originally thought to be necessary. The results so far obtained 

 are very valuable, and appear to indicate that a satisfactory form of silver 

 voltameter is realisable. It is hoped that the publication of the results 

 will take place at the same time as those of the ampere balance. 



An Appendix by Mr. F. E. Smith describes the methods of comparing 

 resistances in use at the National Physical Laboratory, and discusses the 

 sources of error and the accuracy attainable. 



The grant of 25Z. made in 190-5 has been expended in materials for the 

 work on the ampere balance and the silver voltameter. In connection 

 with the latter a large amount of work involving considerable expense 

 remains to be done. For this purpose the Committee ask for reappoint- 

 ment with a grant of 501. They recommend that Lord Rayleigh be 

 Chairman and Dr. R. T. Glazebrook Secretary. 



