56 Messrs. Paterson and Dudding on Estimation of 



authors' results agree very closely with those o£ Forsythe,. 

 and it should further be noted that whilst agreeing with 

 Forsythe, who used the usual optical methods, in the case of 

 comparatively non-selective carbon filaments, they also 

 virtually agree with his results for tungsten, although the 

 latter is admittedly selective. 



Von Pirani and Meyer found values of true temperature 

 which are appreciably higher than Forsythe's, and therefore 

 also higher than by the colour-identity method given here. 

 They are given in column IV. of Table IV., the values being- 

 taken off a curve through Von Pirani and Meyer's values 

 and reduced to the same basis of lumens per watt. A con- 

 siderable amount of this difference appears to be accounted 

 for by the use by Von Pirani of a temperature scale which 

 gives the melting-point of platinum at 1790° C 



Both Von Pirani and Forsythe determined black body 

 temperatures, and added an amount depending on certain 



Sssumptions in order to get true temperatures. It is not 

 ossible to correct Von Pirani and Meyer's figures to make 

 them comparable with those based on a temperature scale 

 which gives the more usually accepted melting-point for 

 platinum, but it is clear that if this could be done Von Pirani 

 and Meyer's figures would agree much more closely with 

 forsythe's. Langmuir does not give details of how he 

 obtained his temperature values, since his paper was not 

 directly concerned with the measurement of temperature. 

 His results differ by about 2 per cent, from the authors' 

 values obtained by extrapolating the curve shown in fig. 3 

 of this paper, using the formula given on p. 50, equations (8 j 

 and (9). 



In order to ascertain if the colour-identity method is correct 

 for substances other than carbon it is necessary to know the 

 true temperature of some glowing filament. The melting- 

 point of platinum is now very generally accepted as 

 (1750 + 20)° C* If a filament of platinum could be 

 gradually raised to the melting-point by an electric current,, 

 and compared at its melting-point against one of the colour 

 standards, the "colour identity" temperature of the platinum 

 could be fixed at the melting-point and compared with the 

 known melting-point of platinum. Platinum is admittedly 

 a selective body. In addition, when radiating in the open 

 its black body temperature determined by the ordinary optical 

 methods (X = 0'650 fju) is some 200° C. lower than the true 

 temperature, and as the colour comparison would be made 



* Burgess & Le Chatelier, ' Measurement of High Temperatures/ 

 p. 492. 



