424 EEroiiT — 1881. 



The repetition of the determination with the original apparatus, by 

 Lord Rayleigh and Professor Schuster in the Cavendish Laboratory, has 

 gone far to supply this requirement. Experiments by another method have 

 also been carried on by Professor G. C. Foster in the Physical Laboratory 

 of University College, London. Some account of these experiments is 

 given in Appendix I. to this report, but the results hitherto obtained 

 can only be regarded as preliminary. With regard to the issue of 

 authorised cojoies of the ohm for general iise, the Committee did not see 

 their way to making arrangements for actual construction of standard 

 coils. They were of opinion that it would be best to limit their action to 

 drawing up a detailed specification for the construction of standard 

 resistance coils, and to arranging for the systematic testing of coils 

 which are certified to them as being made in accordance with this 

 specification, issuing certificates showing their actual resistance. Such a 

 system would be analogous to the system adopted by the Kew Committee 

 for the testing of meteorological instruments at the Kew Observatory, 

 It has not yet been settled by whom this duty should be undertaken. 



An important point of detail connected with the practical construction 

 and use of standard coils has been investigated by Mr. Herbert Taylor. 

 The material adopted by the former Committee for the wire of the 

 standards issued by them, was an alloy of platinum and silver, containing 

 one part platinum to two of silver ; and the same material is very often 

 used for the coils in the ' resistance-boxes ' issued by instrument-makers. 

 One sjaecial reason for the selection of this alloy for the purpose named is 

 its small temperature-rate of variation of resistance, — 0'031% per degree, 

 according to the late Dr. Matthiessen. Mr. Taylor has now found that 

 the rate of variation of resistance of wire made of this alloy depends upon 

 the diameter of the wire, the percentage amount for one degree varying 

 from 00299 for a wire nearly 7 millim. in diameter to 00231 for a 

 wire of diameter 0*168 millim. A detailed account of Mr. Taylor's 

 experiments forms Appendix II. to this Report. 



With regard to standards of GapacUy, the Committee are able, thanks 

 to the zealous co-operation of one of their number, to i-eport somewhat 

 more complete arrangements. Dr. Muirhead having undertaken for the 

 present to make and issue Standard Condensers adjusted in accordance 

 with one whose absolute capacity has been determined by himself and 

 Mr. Hockin. (' Brit. Assoc. Rep.,' 1879, pp. 283 and 285.) 



With a view to testing the permanency of condensers made with 

 mica, paraffined i^aper, or other solid insulators. Dr. Muirhead is also 

 having constructed a large air- condenser. 



In reference to the standard of Electromotive Force, the Committee 

 have had to consider whether this ought to be based upon a particular 

 combination of chemicals, forming a galvanic cell of definite electromotive 

 force, such, for instance, as a Daniell's cell, constructed in a specified 

 manner from materials of guaranteed purity, or the cell introduced by 

 Mr. Latimer Clark (' Proc. Roy. Soc' xx. 444), or whether they should 

 not rather aim at the construction of some convenient form of electro- 

 meter capable of indicating with sufficient accuracy an electromotive force 

 of about a volt. The first plan would be comparable with a supply of 

 ice and boiling water as affording a standard interval of temperature ; the 

 second would be comparable with a thermometer showing the two limits 

 of the standard interval. 



The Committee are not yet prepared to make a final recommendation 



