458 MM. Jamin and Richard on the Laws of Cooling. 



The heated body is no longer a thermometer, but a platinum 

 wire traversed by an electric current ; it does not gradually cool, 

 but, on the contrary, remains at a constant temperature and 

 emits an invariable quantity of heat. Of this the gas takes a 

 portion, is heated, and its pressure augmented to a limit h. "VYhen 

 the stationary condition is reached, the heat 5' taken by the gas 

 from the wire is equal to that which it yields to the sides of the 

 enclosure, or =q. As the latter is known, the former can be 

 found, viz. the velocity of cooling of the wire as a function of its 

 excess t of temperature and of the pressure H of the gas. 



I. To arrive at this it is necessary first to measure the excess 

 of temperature / of the wire. Now we know that the electrical 

 resistance of platinum increases with the temperature. Nume- 

 rous experiments, which will shortly be published, on this sub- 

 ject have been performed in my laboratory by M. R. Benoit : 

 they have shown that, by a valuable exception, the resistance of 

 this metal increases proportionally to the temperature as far as 

 the volatilization of sulphur, and probably beyond; so that the 

 resistances r and r' (at 6 and at t + O) are 



r=r^{\+IJ.e), r'=r^\\+i.{t + e)-]; 

 consequently 



The augmentation of the resistance of the platinum wire is there- 

 fore proportional to its excess of temperature t, which will be 

 known in degrees Centigrade when we have determined rQaud//.. 

 We shall see that this determination is unnecessary, that we 

 need only express t by the values of r'— r, which are propor- 

 tional to it — which merely amounts to changing the thermome- 

 tric scale. 



To measure the increment of resistance 7^ — r, the electric cur- 

 rent is divided into two branches, both of which pass first through 

 copper wires of large section, of little resistance, and equal, 

 wound the same number of times round a difi*erential compass, 

 but in opposite directions. The first branch is then continued 

 by the balloon-wire r\ and the second by a rheostat with a mer- 

 cury cursor (constructed after the pattern devised by Pouillet), 

 and by a second wire identical with the wire in the balloon, but 

 immersed in water at 6 and retaining its resistance r. When 

 the current passes, the needle is deflected ; we bring it back to 

 zero by adding a length of rheostat which compensates and mea- 

 sures the increment of resistance r' — r, and consequently the 

 excess of temperature t. 



II. While the current is circulating in the wire and giving it 

 an excess of temperature r' — r, the pressure of the gas rises to 

 a limit H + A. If we change the intensity of the current, 7^—r 



