316 



The relation between platinum temiwratnre and centigrade temperature 

 from — 1(J0 to 1.1U0° C. is given by the equation 



Vioo / lor 



t — pt = r5 



" 100 



where 6 is a constant depending upon the purity of the platinum. For 

 pure platinum f? is 1.50 and for impure it is somewhat higher. Such a 

 pyrometer is usually calibrated by measuring its resistance at the melting 

 point of ice (0° C), boiling point of water (100° C). and some other tem- 

 perature, s^lc•h as the boiling point of sulphur (444.7° C). 



Temperatures measured on such a pyrometer will agree with the tem- 

 peratures measured on the gas scale in the range to 1,100° C. to within 

 the degree of reproducibility of the latter.'" 



Thermoelectric Pyrometer. — It has been shown by a number of experi- 

 menters that in order to completely calibrate a thermo-couple, point by 

 point comparison is unnecessary, but that three or four known temperatures 

 or tixed points are sufficient. No general equation can be given that will 

 accurately fit all thermo elements, but for most metalf?_. 4t least within a 

 limited region, the relation between the potential dilTereuce in millivolts 

 and the temperature in degl^ees centigrade is sufficiently well represented 

 by the general ciuadratic equation 



e = a + bt + ct= (3) 



where a. b and c are constants that can be determined if three tempera- 

 tures are used. It can easily be determined by experiment how well this 

 formula will hold for any given couple. Three points should be chosen 

 which will cover the region for which the couple is to be used, and a 

 curve drawn through these points. If the curve is nearly a straight line 

 it can be represented by Equa. 3. 



The fixed points are generally the ebullition of water, analine, naph- 

 thaline, sulpliur, etc., or the freezing of such metals as tin, zinc, antimony, 

 copper, silver, gold, etc. The former, with the exception of sulphux, are 

 obtained with less difficulty than the latter, but are of value only for low 

 temperatures. 



For a copper-constantan couple the most convenient fixed points are 

 the fusion temperatiu-e of antimony (030.7° C), zinc (419.4° C), and tin 

 (231.9° C), and for a platinum couple zinc, antimony and copper 

 (1,083° C). 



"Bulletin Bureau of Standards, Vol. 6, p. 196. 



