ON STANDARDS OF ELECTRICAL RESISTANCE. 345 



Report of the Committee on Standards of Electrical Resistance. 



The Committee consists of — Professor Williamson, Professor Wheatstono, 

 Professor W. Thomson, Professor Miller, Dr. A. Matthiessen, Mr. 

 rieeming Jenkin, Sir Charles Bright, Professor Maxwell, Mr. C. W. 

 Siemens, Mr. Balfour Stewart, Dr. Joule, and Mr. C. F. Varley. 



In the present Report it is thought unnecessary again to refer to the objects 

 with which the Committee was appointed, or to recapitulate the arguments 

 for and against the vai-ious systems of standards which have been from 

 time to time proposed. The Committee have seen no reason to alter the 

 conclusions previously adopted, and now propose briefly to state the progress 

 made in the practical development of those conclusions, which may be found 

 expressed at length in the Report for 1863. 



That Report announced the adoption by the Committee of the absolute 

 electromagnetic system of measurement, based on the metre, gramme, and 

 second, with certain modifications to facilitate the practical construction or 

 use of the standards ; and it further stated that in 1863 the absolute resist- 

 ance of a certain German-silver coil had been measured with considerable 

 accuracy. 



No standards based on the 1863 determination were officially issued, in- 

 asmuch as it was felt that a second determination was absolutely required 

 before complete dependence could be placed either on the method employed 

 or on the results obtained. Some coils representing 10 of the British Asso- 

 ciation units, i. e. 10^ absolute units according to the 1863 determination, 

 were made by Messrs. Elliott Brothers, and a set from 1 to 10000 was made 

 from the 1863 determination by Messrs. Siemens and Halske of Berlin. This 

 last set is intended for Col. Douglas, the Superintendent of the Government 

 telegraph lines in India ; and a few of Messrs. Elliotts' coils have been bought 

 by persons who were unwilling to wait for the final experiments by the Com- 

 mittee. None of these coils have been in any way certified as correct by 

 the Committee. 



In order thoroughly to test the value of the experiments made in 1863, it 

 was determined that not only every measurement should be made afresh, 

 but that every element in the experiment should be varied. The experiment 

 consisted essentially in causing a coil, or rather two coils, of copper wire to 

 revolve or spin at a certain definite rate, and in observing the deflection of a 

 magnet, suspended within the coil, by the reflexion of a scale in a mirror 

 attached to the magnet. 



The measurements required in the calculation are the following : — 



a. The mean radius of the coils. 



71. The number of turns made by the copper conductor fonning the coils. 



1. The effective length of the wire. 



b. The breadth of the section of the coU. 



c. The depth of the section of the coil. 



b'. The distance of the mean plane of the coU from the axis of rotation. 

 T. The time of 100 revolutions of the coU. 

 D. The distance of the scale from the mirror. 



2. The scale-reading during each experiment. 



The above measurements are required for what may be called the simple 

 theory, that is to say, the theory omitting aU the necessary corrections arising 

 1864. 2 A 



