ON STANDARDS OF ELECTRICAL RESISTANCE. 311 



The new unit has been actually employed to express the tests of the 

 Atlantic Telegraph Cable. Mr. Varley promises that the unit shall in future 

 be the basis of the coils used by the Electric and International Company. 



Sir Charles Bright promises that the unit shall be exclusively used by the 

 British and Irish Magnetic Telegraph Company. 



A standard has been supplied to the Boyal Engineers at their request. The 

 head of the Telegraph Department in India has introduced the unit, and 

 there is little doubt that the British Colonies generally will adopt it. 



More time will certainly be required to introduce it on the Continent. The 

 Erench Government has taken no steps to ensure its introduction, but 

 M. Blavier, the official editor of the ' Annales Telegraphiques,' has promised 

 his cordial support to the Committee. The Austrian Government has pro- 

 mised to use the coils experimentally, and the German gentlemen to whom 

 coils were given have promised to give their best assistance. 



Coils "have also been bought by the managers of two large telegraphic 

 establishments in Switzerland, at Neuchatel, and Zurich. There is there- 

 fore reason to hope that the unit may come into extensive use. 



"When standard galvanometers, Lcyden jars, and electrometers are issued, 

 all forming part of one coherent and necessary system, it is probable that the 

 B.A. unit will be found so much more useful than any other as to supplant 

 them entirely. Until these further issues take place, it will only be adopted 

 either by men who can understand the advantage given by it in calculation, or 

 by electricians who feel confidence in the recommendations of your Committee. 



With a view to experiments which will allow of these further issues of 

 electrical units, a large electrodynamometer has been designed, and is nearly 

 complete. Graduated Ley den jars, with air as the only dielectric, have also 

 been designed and are nearly ready for use. An apparatus for the determi- 

 nation of the quantity called v in Appendix C of the 1363 Keport is in the 

 same condition. Prof. W. Thomson has for some time had ready apparatus 

 for absolute measiu-ements of electrical effects, but his connexion with the 

 Atlantic Cable has suspended his work. Dr. Joule promises fresh measure- 

 ments of the mechanical coefficient of heat, and has only been delayed by 

 the want of experiments which other members of the Committee must pre- 

 viously complete. 



In conclusion, the Committee are at last able to report one positive result, 

 but they feel that much more remains to be done. 



Appendix A.— On the Construction of the Copies of the B.A. Unit. 

 Bij A. Matthiessex, F.B.S., and Mr. Charles Hockin. 



The standard coil used in the experiments at King's College, described in the 

 Report of your Committee for 1864, was put into our hands about last 

 Christmas, in order that unit-coils representing a resistance equal to ten 

 million metres per second in Weber's electro-magnetic system might be made 

 from it. 



Since that time several unit-coils have been made and issued. 



We propose to state the method by which these coils were made, and the 

 reasons for choosing the particidar alloy which has been adopted for the con- 

 ductor. The alloy referred to is composed of 66 per cent, of silver and 33 

 of platinum. 



This alloy possesses many properties which fit it for the use to which it has 

 been put. 



As to its electrical properties : — ■ 



