Thermal Radiation in Absolute Measure. 



273 



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"^ of special construc- 

 tion ; 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 resist- 

 ance and temperature were simultaneously determined. A consider- 

 able number of readings between 15° C. 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 inter- 

 mediate points. 



In one respect, the determinations, an account of which is given in 

 the present paper, are not perfectly satisfactory. We have not been 

 able to take account in a proper way of the temperature of the enclosing 

 envelope. In order to be able to see the condition of the wires, and in 

 particular to observe their appearance when they l)ecame luminous, 

 glass envelopes were used in these experiments ; and owing to the 

 nature of the arrangements and the method of experimenting, it was 

 not found possible to immerse the glass envelopes in a cooling bath. 

 Consequently the glass became more or less heated during the experi- 

 ments, and the heating was unequal in the cases of the bright wire and 

 the sooted wire. It has already been pointed outf that the proportions 

 in which the radiations of longer period and shorter period are present in 

 the total radiation depends on the radiating surface, other things being 

 the same. In the case of the sooted wire, the quantity of long-period 

 radiation is, in proportion, far in excess of that proceeding from a 

 bright metallic polished surface. Consequently, with the same total 

 electric energy supplied to both wires, the glass tube containing the 

 sooted wire became much hotter than the tube containing the bright 



It has also been pointed out| that with a substance like glass, con- 

 ducting badly and somewhat diathermanous, it is impossible to tell how 



* J. T. Bottomley, " On a Practical Constant-volume Air Thermometer, ' Proc. 

 Roy. Soc. Edin.,' December 19, 1887, and =PhiL Mag.,' August, 1888. This 

 thermometer lias proved perfectly satisfactory ; and the separation of the volume 

 gauge ana pressure gauge make it extremely convenient for applications of the 

 kind referred to in the text. 



t ' Phil. Trans.,' A., 1887, p. 450. 



X Loc. cif., p. 444. 



