ON STANDAEDS FOR USE IN ELECTRICAL MEASUREMENTS. 71 



annealing on the specific resistance of wires and on the temperature- 

 coefficient to be much greater than has hitherto been commonly supposed. 

 The following are examples of some of the results obtained : — 



German Silver. — Wire drawn to be extremely hard and brittle. The 

 percentage variation of resistance, for 1° between 13° and 100° C, was 

 0'0296. After annealing, the percentage variation of the same wire was 

 0-0421. 



Steel. — "Wire, 0'025 inch diameter, thoroughly hardened, and then 

 tempered in paraffin wax at 230° C. : — 



Percentage variation of resistance for 1°, 0'267. 



Same wire annealed ; percentage variation for 1°, 'SIG. 



At 9° C, the ratio of the absolute resistance of this wire in the hard 

 state to that of the same wire when annealed was 1"229. 



Platinum- Silver Alloy. — A piece of wire made from a particular bar 

 of the alloy was hardened by being drawn down through a couple of 

 holes of the draw-plate. In this state the variation of resistance was 

 0"0255 per cent, per degree. After annealing in the ordinary way, the 

 variation of resistance per degree was 0'0258 per cent. The same wire 

 was next placed in an iron tube which was filled up with sand and left 

 all night in the fire. After this treatment, the percentage variation of 

 resistance per degree was 00344. 



Platinum- Silver Alloy, another specimen. — A wire from a second 

 bar of the alloy was annealed at a very high temperature and left to soak 

 in the fire and cool slowly, as in the last-mentioned experiment. The 

 variation of resistance was now 0'095 per cent, per degree, and the wire 

 was as soft as pure silver and very fragile. After being heated to red- 

 ness and quenched in water, the corresponding variation of resistance of 

 the same wire was 0076 ; and when the wire had been drawn down 

 through two or three jewel-holes, it was 0'0732. 



These results indicate a connection between the temperature-coefficient 

 of wires and their degree of hardness, and tend to reopen the question as 

 to the most trustworthy material for a permanent standard of resistance. 

 The Committee understand that Mr. Taylor will continue his experiments 

 with the co-operation of Dr. Muirhead. 



The Committee are pleased to be able to report that there is a pro- 

 spect that Lord Rayleigh may be able to organise, at the Cavendish 

 Laboratory, Cambridge, a system of testing resistance-coils and issuing 

 certificates of their correct value at a specified temperature. 



As stated in the Report presented last year, Dr. Muirhead has con- 

 sented, at the request of the Committee, to issue standards of capacity 

 upon his own responsibility. 



The Committee regret that they are not able to report any progress 

 towards the construction of a standard of Electromotive Force. 



They are unwilling to conclude without expressing their deep sense of 

 the loss which not only they, but all friends of physical science, have 

 snffisred in the death of one of the most valued of their colleagues, Mr, 

 Charles Hockin. 



