46 Messrs. Paterson and Dudding on Estimation of 



(6) The following considerations show to what extent a 

 temperature, determined as above, may be regarded as the 

 temperature of the main glowing part of the filament. If 

 there were no cooling at the ends of a filament it would be 

 equally bright for the whole of its length. The cooling, 

 however, as shown by Hyde, Cady, and Worthing* is appre- 

 ciable, but it must be remembered that the colour of the 

 light is governed by the part of the filament giving off most 

 light. The ends of the filament give off actually very 

 little light because the amount emitted falls off according to 

 a very high power of the temperature (T 12 to T 2 °). Hence 

 the effect of the dulled ends of the filament on the colour of the 

 total light from it is exceedingly small. In an actual case t 

 the total light emitted below the point where the filament 

 began to become measurably dull was only 5 per cent, of the 

 whole, and a large percentage of this amount differs only 

 ■very slightly in colour from the light emitted by the remainder 

 of the filament. 



The following measurements, Table II., were made of the 

 total effect on the measured temperature due to end cooling 

 by determining the " colour identity " temperature of the 

 central portion of the filament only and comparing it against 

 that of the whole filament, including the cooled ends. 



Table II. 



Lamp. 



Volts. 



Temperature of black body for 

 colour identity. 



Whole filament. 



Centre of filament. 



No. 11 | 



200 

 110 



54 



2,245 



1,885 

 1,556 



2,255 



1,898 

 1,570 



No.9 \ 



105 

 60 

 40 



2,280 

 1,920 

 1,710 



2,290 

 1,930 

 1,722 



Motor headlight. J 

 16 volts, 50 watts 1 





1,486 

 2,014 

 2,240 



1-508 

 2,043 



2,285 





Lamp No. 11 was an ordinary 200 volt, 20 watt tungsten 

 lamp, and No. 9 was rated for 100 volts, 55 watts ; the 

 differences of filament diameter and distance between supports 



* Amer. 111. Eng. Soc. Trans. 6, pp. 238-257. 

 f See Hyde, Cady, and Worthing, loc. cit. 



