16 The Eev. T. E. Robinson on the Relation between the Temperature of 



For this metal the coefficients of expansion are a = log"' (5.03826); 

 j3 = log"' (1.40598). As the resistance was measured by the rheostat alone, 

 the correction cA'-C' need not be applied, and the equations are 



33.77 = a + ?< X 215.40, 

 45.90 = a + 6x 558.25. 

 Hence, 



a = 26.37 ; b ^ 0.0348 ; - = 0.00133, 



a 



The value of - is so near that of platinum as to make it an object of interest 



to ascertain whether tlie same equality prevails in other metals. The difference 

 of conducting power in copper and platinum appears very strikingly here. At 

 32° the resistances are as 1:3.1; but in Nos. 46 and 39, when the current is 

 the same, as 1 : 7.6. It may also be remarked, that these constants give, as the 

 probable values of the correction cA'^C^, quantities closely agreeing with those 

 computed by the values of c given above. 



The facility of bringing the wire of the pyrometer to a given temperature, 

 and maintaining it, makes it a convenient source of light in photometers ; but 

 as the heat is not uniform along it, it seems worth inquiry according to what 

 law it varies. Each section of the wire is traversed by the same current, and, 

 therefore, under similar circumstances, would be equally heated : but the heat 

 thus excited is dissipated by three cooling agencies. The first of these is radia- 

 tion, which, though lessened by the bright metallic surface of the pyrometer, 

 below what it would be in free space, is still very powerful. The second is 

 the presence of air ; and the third the conducting power of the wire itself, by 

 which a portion of the heat escapes to tlie metallic supports which attach it 

 to the instrument, in this case chiefly to the lower one. As long as their 

 combined effects are inferior to the heating power of the current, the tempe- 

 rature must increase : while doing so, however, the resistance also increases, and 

 with it (as shall be immediately shown) the heating power, the current being the 

 same. On the other hand, the cooling causes also augment in energy, and in 

 a still higher ratio. An equilibrium of these powers is therefore attained, to 

 whicli belongs the thermic state shown by the pyrometer : it is, therefore, ex- 



