High Temperatures by the Method of Colour Identity. 41 



the approximate watts per mean horizontal candle of the 

 glow-lamps is also given, since this is the more familiar desig- 

 nation, but it is not rigorous on account of the varying ratios 

 of mean horizontal to mean spherical candle-power to be 

 found in different lamps. It is to be noted that in the deter- 

 mination of the total light emitted from the glow-lamps the 

 light which is obscured by the cap of the lamp is counted as 

 being radiated and not absorbed, but in any case this light 

 is less than 1 per cent, of the total for filaments of ordinary 

 form. 



The objects of the measurements are: — 



(a) To find the relation between the various values of 

 lumens per watt for tungsten and carbon filament lamps and 

 the corresponding temperatures of a black body on the basis 

 of colour identity ; 



(b) To ascertain to what extent the temperatures so measured 

 represent those of the principal parts of the glowing fila- 

 ments, having regard to the cooling effect of the filament 

 supports ; 



(c) To find laws connecting lumens per watt and corre- 

 sponding "colour identity" temperature for tungsten and 

 carbon filaments. 



Evidence is given later in the paper to show that there 

 seems to be justification for the assumption that the colour- 

 identity method at any rate in certain cases gives within 

 narrow limits a measure of the true filament temperature. 

 This assumption is therefore made in what immediately 

 follows here, and it will he seen that the results which follow 

 from this assumption, whilst not proving its validity, are in 

 agreement with those of Forsythe, who determined tempera- 

 tures by more orthodox methods. 



(a) A number of carbon and tungsten filament lamps were 

 selected for the measurements. The carbon lamps had both 

 flashed and unfinished filaments. The tungsten lamps had 

 squirted and drawn filaments of different diameters and 

 lengths, so that the effect of the cooling of the ends by the 

 leading-in wires if appreciable might be observed. 



All the lamps were measured for lumens per watt at 

 different voltages up to the highest they were capable of 

 standing without deterioration. They were then compared 

 for identity of colour against the colour standards, and in 

 this way the temperatures of a black body were determined 

 which corresponded with the various values of lumens per 

 watt. The results are given in Table I. and plotted in fig. 3. 



