1884.] of Conductors through ivltich a Current is passing. 181 



resistances of BC when, respectively, the weak current and the 

 stronger current are applied ; also let a lt a 2 and b lt 6 3 he the corre- 

 sponding respective lengths of the shorter and longer parts of the 

 branch A'B'C. 

 Then 



(1) cir-j ::!:&! 



and (2) c : r 3 : : a 2 : 6 2 



TT **9 a l ^2 



Hence ,*=-L^* 



r i a A 



Let t be the temperature of the air of the room where the experi- 

 ment is made, and let it be assumed that the temperature of the wire 

 BC is the same as that of the air during the first experiment with the 

 weak current ; let Q be the temperature of the wire when the stronger 

 current is passing. Let at and a e be the specific resistance of these 

 substances at these temperatures. 



Then r J=- 9 



"i fft 



the change in dimensions of the wire due to heating being left out of 

 account. The values of at and a Q can be found, as has been remarked 

 already, for copper and for several other substances from well-known 

 tables and experiments. 



My first experiments, after some preliminary trials had been made, 

 were on a series of copper wires, all of them cut from the same piece, 

 and of the same length, but some of them bare and others covered 

 with different insulating coatings. The wires were about No. 21 of the 

 old B.W.G., 0'81 millim. in diameter and half a metre in length. The 

 resistance of each, with only as much current passing as sufficed for 

 the ordinary application of Wheatstone's bridge, was 0'0183 B.A . unit. 

 These were tested by the potential method described above. Eight 

 wires were taken, and their surfaces were prepared as follows : 



A. Surface amalgamated with tin and mercury. 



B. Surface amalgamated with mercury by rubbing on mercury 



nitrate and washing. 



C. Bare surface (dim). 



D. Covered with cotton and shell-lac. 



E. Cotton -covered. 



F. Shell-lac varnish. 



G. Bare, polished with fine emery. 

 H. Thick coating of lampblack. 



These wires were tested successively with currents of 1"2 amperes, 



