1884.] of Conductors through which a Current is passing. 185 



The current was in every case 10 amperes. Increase in the 

 numbers indicates increase in the resistance of the tested wire, and 

 therefore increased temperature ; while decrease in the numbers 

 quoted shows decrease in the temperature. The change in the 

 permanent temperature is not more than two degrees ; and the 

 result of these experiments may be briefly stated to be this, that 

 there was exceedingly little effect in the way of keeping up the 

 temperature of a No. 20 B.W.G. copper wire, about 1 millim. in 

 diameter, by covering it up with paper and Brunswick black till it 

 attains an external diameter of more than a centimetre and a half. 



A large number of experiments were tried of a kind similar to 

 the one just quoted, and all of them giving similar results for wires 

 of similar dimensions. I propose with improved arrangements 

 which I have now at command, to obtain numbers which shall 

 be more accurate than those which I have yet obtained ; and to 

 extend the experiments to the case of wires of larger diameters. 



I have also made some preliminary experiments on copper wires of 

 various diameters without solid covering, cooling in air and in 

 vacuum. In commencing these experiments I used as a formula 

 for controlling my results an expression derived as follows : 



A current passing through a given wire produces heat of which the 

 amount, according to the well-known formula of Joule, is given by 



H=C 2 R/J. . . . . j\ . . . (1), 



where C is the current, R the resistance, J Joule's equivalent, and H 

 the quantity of heat produced per second, each being reckoned in 

 (J.G.S. units. Let I be the length of the wire experimented on, and 

 d its diameter ; and let a t be the specific resistance of the substance 

 at the temperature t. Then 



-p _ <TI x I _ 4>fftl 



~1^2~~^2~ 



Hence from (1) .$,- 



Now let H' be the quantity of heat lost by the wire by emission 

 from the surface ; let e be the emissivity ; and let 6 be the tempera- 

 ture of the surroundings within which the wire is cooling. 



Then 



H'=ircW.e(<-0). ........ (3). 



But when a permanent temperature is attained H must equal H', and 

 hence we have from (2) and (3), 



~ 



