COMPARISON OF RESISTANCES. 



metals, for their resistance varies less with the temperature. While 

 the coefficient of variation is 0*0039 for copper, it is only 0*00444 

 for argentan, and 0*00031 for an alloy of two-thirds silver and one- 

 third platinum. 



Fig. 179 represents the form of standard which was adopted by 

 the committee of the British Association in 1865. The wire is an 

 alloy of platinum-silver : it is covered with a double layer of silk, 

 and varnished with shellac. After having been doubled, it is folded 

 as a spiral, which is then placed between two concentric cylinders of 

 brass, and immersed in a mass of paraffine filling the interval of the 

 two cylinders. The two ends of the wire are soldered to two stout 



Fig. 179. 



copper wires bent twice at right angles, and the amalgamated ends of 

 which dip in mercury cups. 



The cylinder may be placed in ice or in water; a thermometer 

 placed in the central tube gives then the temperature. Although 

 this arrangement gives a large surface of contact with water, the bad 

 conductivity of paraffine always leaves great uncertainty as to the 

 true temperature of the wire, unless the temperature of the bath has 

 been kept constant for several hours. 



The form adopted by Siemens is represented in Fig. 180. The 

 wire, which is of argentan, and covered with silk, is coiled as a 



