54 Messrs. Paterson and Dudding on Estimation of 



Tungsten Lamps. 



Similarly the equations for tungsten filaments are changed 

 when cooling is allowed for, from 



logW = C 8 + 5\LlogT, (23) 



log^=C 4 -5-llogT-1851og(l+^) . (24) 



an 



to logW=Cs-0-700 + 5-31ogT, (25) 



and logJ± =C 4 + 0-703-5-3 logT-185 log (l + -^\ {26) 



and the same phenomenon is observable as for the carbon 

 filament lamps. 



By comparing equations (13) and (15) and (25) and (26) 

 respectively, and remembering that (13) and (25) deal only 

 with the watt relationship, omitting lumens altogether, it 

 will be seen that practically all the change produced by 

 cooling is in the watts. That means that the loss of lumens 

 at the ends of a filament is virtually equal to the gain in 

 lumens due to the centre of the filament (as shown in Table 

 II.) being at a slightly higher temperature than that ascribed 

 by the colour-identity method to the filament as a whole. 



The fact that temperatures based on the colour-identity 

 method and covering so wide a range are thus found to fit in 

 so well with fundamental theory seems to afford presumptive 

 evidence that the colour-identity method gives values of 

 temperature which are accurate at least from the relative 

 point of view, and affords further experimental support for 

 the conclusions which are arrived at on pages 35-37 as a 

 result of a general discussion of the phenomena underlying 

 colour identity. 



3. Temperature of Lamp Filaments and of Melting 

 Platinum. 



The foregoing experiments assume that a temperature 

 ascribed to a filament by comparing the hue of its total 

 radiation with that of a black body of known temperature 

 approximates to the true temperature of the filament although 

 the latter is glowing under open radiation conditions. If 

 this approximation can be shown to be a very close one, the 

 method might be of considerable use in certain branches of 

 practical pyrometry. In what follows enough evidence is 

 given of the correctness of the temperatures determined by 

 the colour-identity method to justify the assumption for 

 certain substances, and to warrant a more complete investi- 

 gation of the subject. 



