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XXXIX. A Comparison of the Standard Resistance-coils of the 

 British Association with Mercury Standards constructed by 

 M. J. E. Benoit of Paris and Herr Strecker of Wiirzburg. 

 By E. T. Glazebkook, M. A., F.R.S., Fellow and Assistant 

 Tutor of Trinity College, Cambridge*. 



IN accordance with the resolution of the Electrical Congress 

 held in Paris in 1884, the Electrical-Standards Committee 

 of the British Association decided to have made a series of 

 resistance-coils in terms of the Legal Ohm ; and the work of 

 testing and comparing these coils was entrusted to me. In 

 Paris, at the request of the Minister for the Post Office and 

 Telegraphs, M. J. B. Benoit undertook the same task. An 

 account of his experiments is published in the Journal de 

 Physique for January 1885. 



M. Benoit started ab initio, and constructed a series of 

 glass tubes, the electrical resistance of which, when filled with 

 mercury, can be calculated from their dimensions. Four of 

 these were made, each having a resistance of about 1 ohm. 



The plan adopted by the Committee was different. The 

 specific resistance of mercury in terms of the British-Associa- 

 tion unit has been carefully determined by Lord Eayleigh 

 and others. 



For the purpose of constructing Legal Ohms, it was decided 

 to adopt a number for this quantity founded on their experi- 

 ments, and then work from the British- Association standards. 

 To adopt the other course w r ould only have been to repeat at 

 some trouble the experiments of Lord Eayleigh, M. Mascart* 

 Herr Strecker, and others. 



M. Benoit made a number of copies of his mercury-standards, 

 and after reading his paper, I wrote and asked him to send 

 me one or more of these, that I might compare them directly 

 with the coils which had by that time been constructed for 

 me by Messrs. Elliott Bros. M. Benoit replied most cour- 

 teously, and sent me, about Easter, three of his copies. The 

 object of the present paper is to give an account of the com- 

 parison of these with the standards belonging to the British 

 Association. 



M. Benoit's copies were mercury-standards. Each con- 

 sisted of a glass tube bent several times, A A, fig. 1. The 

 ends of this tube are ground flat and pass into two glass cups, 

 B, B, which for most of their length are considerably wider 

 than the tube, but taper down at their lower ends. The tubes 

 are connected to the cups by short bits of india-rubber tubing, 



* Communicated by the Physical Society : read May 23, 1885. 



