168 Prof. W. E. Ayrton and Mr. H. Kilgour, [Nov. 19, 



wire through which a steady current was flowing and from which heat 

 was lost by radiation, convection, and conduction, and it was further 

 necessary to improve on the calculation one of us had published on 

 this subject in the ' Electrician ' for 1879, by taking into account the 

 fact that the emissivity, as well as the thermal and electric conducting 

 power, of the wire differed at different points in consequence of the 

 difference of temperature. 



Until we had completed the experiments described in this paper 

 we could, of course, only employ in this calculation values that we 

 had guessed at as being something near the truth for the emissivity 

 of platinum wire for different diameters and at different temperatures. 

 Hence, after the completion of the experiments, we took up the 

 mathematical investigation again, substituting for the emissivity such 

 a function of the diameter of the wire and the temperature of the 

 point as we had experimentally found it to bB. Section IV of the 

 paper contains the investigation by which we finally arrived at the 

 calculated distribution of temperature along the wire, and we have to 

 express our sincere thanks to Professor Henrici (whom we consulted 

 as to the best method of practically solving the rather complex 

 differential equation arrived at) for the warm interest that he has 

 taken in the mathematical treatment of the subject, and for the many 

 suggestions which he has made, and which have enabled us to arrive 

 at the mathematical solution given in the paper. 



Each wire to be tested was stretched along the axis of a water 

 jacketed cylinder 32*5 cm. long, the inner surface of which was 

 blackened and* kept at a constant temperature by a stream of water 

 flowing through the jacket. The rate at which heat was lost by any 

 one of the wires was measured by the product of the current passing 

 through it into the P.D. (potential difference) maintained between its 

 ends, while the ratio of the P.D. to the current gave the resistance of 

 the wire and, therefore, its temperature. Experiments were in this 

 way made with various currents flowing through each of -the nine 

 "w ires. 



As the variation of resistance with temperature is known to vary 

 with different specimens of platinum, experiments were separately 

 made to determine the actual law of variation of resistance with 

 temperature up to 300 C. for each piece of wire that had been 

 employed in the emissivity experiments. 



In this later determination various thermometers were used, and 

 the subsequent comparison of these thermometers with a Kew standard 

 thermometer involved a vast amount of labour, from the fact that it 

 is, or at any rate was not possible three years ago, to purchase from 

 the Kew Observatory a standard thermometer reading from, say, 200 

 to 300 C., with a short, wide chamber at the base in which the 

 mercury expanded below 200 C, All that could be obtained was a 



