ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 



435 



to have the greatest available cross section ; each is held rigidly in 

 position by a screw passing radially through the ebonite stopper, and 

 they ai'e insulated one from the other by an air space, through which pass 

 the wires for the galvanometer connections, the interstices being after- 

 wards filled with shellac. 



The ends of the wire to be tested are soldered to the upper extremities 

 of the copper i-ods. When the wire is short and thick, it is covered with 

 silk, bent double, and passed into the tube ; when thin, it is wound on a 

 glass rod, and the whole coated with silk ribbon. The space between 

 the wire and tube is filled with lead shot of the smallest size, and a ther- 

 mometer is inserted in the top of the tube, which is then filled quite up 

 with shot. 



It was found that the shot had an exellent effect in preventing minute 

 oscillations of temperature, due to draughts or similar causes. 



The battery used was one large Grove cell ; the galvanometer, a 

 ' dead beat ' by White, having a resistance of 1'4 ohms. 



In most cases the absolute resistance of the specimens of alloy was 

 determined as well as the temperature-coefficient. 



For the bars this was done by comparison with a rod of pure lead of 

 known dimensions. 



The wires were compared directly with bobbins of known resistance. 



The sectional area of the bars and wires was always calculated from 

 their weight, specific gravity, and length. 



All bars and wires were annealed before making the electrical 

 observations. 



The following table shows the resistance and temperature-coefficient 

 of the first sample of the alloy when in the original bar shape, and when 

 drawn down to wire of various gauges. 



A complete analysis of this alloy from the end of the rod was made 

 for me by Messrs, Johnson and Matthey. The composition was as 

 follows : 



Platinum 32-95 



Silver ' 66-65 



Iridium 0-08 



Rhodium 001 



Iron . ....'.".'.'.' ." 0-01 



Lead, with mimite trace of copper 001 



Loss . 0-34 



100-00 

 F F 2 



