High Temperatures by the Method of Colour Identity. 57 



under open radiation conditions, the experiment should be a 

 crucial one for proving if the colour-identity method gives 

 true temperatures. 



The only precaution necessary is to take a fair length of 

 platinum wire and use only the central portion, so that the 

 cooling of the ends of the wire by the leading-in terminals 

 shall not influence the determination. Lengths of No, 25 

 gauge wire were used, 13 cm. long, of which all but the 

 centre 5 cm. was screened off. Simultaneous comparisons 

 were made with both carbon and tungsten colour standards. 

 The current in the platinum wire was slowly raised, and that 

 in the colour standards increased so that identity of colour 

 was always maintained in the photometer up to the melting- 

 point of the platinum. The colour standards were arranged 

 each with a photometer head on either side of the platinum 

 wire, and no difficulty was found in maintaining colour 

 identity to the point at which the platinum melted. 



Table V. gives the results of all the 15 determinations. 



Table V. 



Experiment. 



Temperature of platinum at melting-point by colour- 

 identity method °0. 



With carbon filament 

 colour standard. 



1,752° C. 

 1,746° 

 1,727° 

 1.757° 



1,761° 

 1,769° 

 1,755° 

 1,763° 



1,759° 

 1 .727° 

 1,7M7° 

 1,747° 

 1.764° 

 1.757° 



With tungsten filament 

 colour standard. 



1,765° C. 



1,770° 



1,751° 



1,7(55° 



1,747° 



1,784° 



1,779° 



1,784° 



1,782° 



1,782° 



1,765° 



1.769 3 



1,782° 



1,789° 



1.789° 



i 



s:::::::::::.:::::: 



4 



C) 



7 



8 



9 



10 



11 



12 



13 



14 



15 





Mean 1,750° C. 



1,773° 0. 



1 



The mean result gives the melting-point of platinum as 

 1750° 0. by the carbon filament lamp and 1770° 0. by the 

 Tungsten lamp, a result so near to the accepted value of 

 1750° C* as to afford strong evidence of the reliability of 



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

 p. 492. 



