The v null Radiation in Absolute Measure. 



547 



second wire. Usually these pairs of readings were identical, 

 or nearly so, as no reading was taken until after the strong- 

 current had been passing through the circuit for sufficient 

 length of time to allow the temperature of the whole to become 

 perfectly steady. Generally speaking, five minutes or more 

 was allowed for this purpose. Lastly the current was again 

 read, and the zero of the potential galvanometer noted. 



The readings detailed above enabled us to calculate the 

 current passing through each wire and the resistance in that 

 wire. The length and cross-section of each of the platinum 

 wires (practically identical) were also known. Thus the energy 

 lost by radiation per square centimetre per second, C 2 R/JS, 

 could be calculated ; C being the current, R the resistance 

 and 8 the surface of the ra liating wire, and J being the 

 dynamical equivalent of heat, all in absolute measure. 



The measurement of the electric resistance of the wires also 

 enabled us to calculate the temperatures of the wires by means 

 of the results of a separate determination of the electric 

 resistances, at different temperatures, of the wires themselves. 



In a former paper*, the precautions and difficulties con- 

 nected with the determination of change of resistances of 

 platinum wires with temperature have been fully discussed. 

 In the present case each platinum wire, after having been used 

 in the radiation experiment, was wrapped round the bulb of 

 an air thermometer f of special construction ; the bulb and 

 wire were then packed in asbestos wool, and placed in the 

 laminated copper heating-jacket described and figured in the 

 paper just referred to. The jacket was heated by means of 

 one of Fletcher's powerful " solid flame " burners, by means 

 of which it could be kept for any length of time almost 

 absolutely steady, at any temperature below the softening 

 point of glass. 



By means of stout copper electrodes the platinum wire was 

 made one of the branches of a Wheatstone balance, and the 

 electric resistance and temperature were simultaneously 

 determined. A considerable number of readings between 

 15° 0. and 350° C. were taken, and from these an empirical 

 formula was constructed, or a curve drawn to represent the 

 relation between temperature and pressure at all intermediate 

 points. 



* J. T. Bottomley, Phil. Trans. 1887. 



f J. T. Bottomley, " On a Practical Constant-volume Air Thermometer," 

 Proc. Roy. Soc. Edin., December 19, 1887, and Phil. Mag., August 1888. 

 This thermometer has proved perfectly satisfactory ; and the separation 

 of the volume-gauge and pressure-gauge make it extremely convenient for 

 applications of the kind referred to in the text. 



