High Temperatures by the Method of Colour Identity. 39 



is a Lummer-Brodhun photometer head. This photometer is 

 not used to compare the intensities o£ the two sources of light 

 A and C, but to determine when the hue of their radiations 

 is identical. Two optical pyrometers were used of the 

 Siemens and Fery types, by means of which the temperature 



Diagram showing arrangement of Apparatus used to obtain the 

 Relation between the Current in Carbon and Tungsten Filament 

 Lamps, and the Temperature of a Black Body at Equality of Hue 

 of their Radiations. 



of the black body was determined before and after each 

 colour determination by the photometer. Complete deter- 

 minations were made on two separate occasions and the mean 

 result taken — every photometer and pyrometer setting being- 

 made on each occasion by two observers. The furnace was 

 first run at a relatively low temperature, and the electric 

 heating adjusted so that it would remain at a constant tempe- 

 rature sufficiently long for both photometric and pyrometric 

 readings to be taken. The hue of the light from the electric 

 lamp C was varied by means of rheostat H until there was 

 exact identity of colour in the photometer. In doing this it 

 is, of course, necessary to place the photometer so that there 

 is also equality of brightness. 



Two or three settings were made by each observer before 

 and after which the pyrometers were read. The same process 

 was followed in a series of increasing temperatures up to 

 about 2200° 0. — the maximum temperature to which the 

 furnace was carried in these experiments. Two electric lamps 

 were calibrated in this way, one having a carbon and the 

 other a tungsten filament. These two lamps calibrated in 

 the above manner form intermediate standards of colour 

 against which any glow-lamp can be matched, and the tem- 

 perature of the black body fixed to which the colour of its 

 light corresponds. The temperatures so determined are on 



