316 
The relation between platinum temperature and centigrade temperature 
from — 100 to 1.100° C. is given by the equatien 
100 100 
where disa constant depending upon the purity of the platinum. For 
pure platinum 0d 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, such 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 0 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 fixed points are sufficient. No general equation can be given that will 
accurately fit all thermo elements, but for most metals, at, least within a 
limited region, the relation between the potential difference in millivolts 
and the temperature in degrees centigrade is sufficiently well represented 
by the general quadratic equation 
e=a+btict (3) 
where a, b and ¢ 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, sulphur, ete., or the freezing of such metals as tin, zinc, antimony, 
copper, silver, gold, etc. The former, with the exception of sulphur, 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 temperature of antimony (630.7° C.), zine (419.4° C.), and tin 
(231.9° C.), and for a platinum couple zinc, antimony and copper 
(1,083° C.). 
10 Bulletin Bureau of Standards, Vol. 6, p. 196. 
