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III. The Estimation of High Temperatures by the Method 

 of Colour Identity. By Clifford C. Paterson and 

 B. P. Dudding, A.R.C.Sc. (From the National Physical 

 Laboratory.) * 



Synopsis, 



1. T)RELIMINARY experiments are described on the 

 JL method of " colour identity " adapted to the estima- 

 tion of the temperature o£ incandescent substances such as 

 metal or carbon radiating in the open; by this method the 

 " true " temperature of certain bodies as distinct from their 

 " black body " temperatures can be arrived at with a very 

 fair degree of accuracy. 



2. By the colour-identity method the total luminous 

 radiation (white light) from a black body is made identical 

 in colour with that from the incandescent metal under ex- 

 amination by adjusting the temperature of the black body 

 until there is colour identity in the field of a Lummer- 

 Brodhun photometer. 



3. Comparisons are made of the results so obtained with 

 those obtained by other methods, and the colour-identity 

 method is shown to give the correct result for melting 

 platinum. 



4. Formulae are deduced, based on the fundamental theories 

 of energy radiation and the sensitivity of the eye, connecting 

 the temperature of carbon and tungsten filaments with their 

 lumens per watt, and it is shown that these expressions hold 

 from the lowest to the highest values of lumens per watt. 



5. It is shown that the colour-identity method of deter- 

 mining filament temperatures is practically independent of 

 the cooling at the ends of the filaments of ordinary lamps. 



6. An explanation is given of the principal factors and 

 limitations of the colour-identity method, in which it is shown 

 that accurate results should be obtained so long as the bodies 

 under consideration act as "grey" bodies throughout the 

 visible spectrum, and that there will be a tendency to error 

 to the extent that they depart from the grey body condition 

 in the visible spectrum. 



7. The colour of the radiation from melting platinum is 

 shown to be the same as that from a carbon filament lamp 

 operating at 2*6 5 lumens per watt, or 4*7 5 watts per mean 

 spherical candle, or approximately 3*8 watts per mean hori- 

 zontal candle. 



* Communicated by the Authors. From the Proc. Phys. Soc. London, 

 April 15, 1915, p. 230. 



