Prof. H. L. Callendnr on Platinum Thermometry. 195 



■which it contained, so far as they relate to the subject of 

 platinum thermometry. 



(1) It was shown that a platinum resistance-thermometer, 

 if sufficiently protected from strain and contamination, was 

 practically free from changes of zero over a range of 0° to 

 1200° C, and satisfied the fundamental criterion of giving 

 always the same indication at the same temperature. 



(2) It was proposed to use the platinum thermometer as a 

 secondary standard, the temperature pt on the platinum scale 

 being defined by the formula 



^ = 100(R-E°)/(R / -E°), .... (1) 



in which the letters R, R°, R / si and for the observed resist- 

 ances at the temperatures pt, 0°, and 100° C. respectively. 



(3) By comparing the values of ' pt deduced from different 

 pairs of specimens of platinum wires, wound side by side and 

 heated together in such a manner as to be always at the same 

 temperature, it was shown that different wires agreed very 

 closely in giving the same value of any temperature pt on the 

 platinum scale, although differing considerably in the values 

 of their temperature-coefficients. (See below, p. 209.) 



(4) A direct comparison was made between the platinum 

 scale and the scale of the air-thermometer by means. of several 

 different instruments, in which the coil of platinum wire was 

 enclosed inside the bulb of the air-thermometer itself, and so 

 arranged as to be always at the same mean temperature as the 

 mass of air under observation. As the result of this com- 

 parison, it was shown that the small deviations of the platinum 

 scale from the temperature t by air-thermometer could be 

 represented by the simple difference-formula 



D = t-pt = d(tl\00-l)tll00, .... (2) 



with a probable error of less than 1° C. over the range 0° to 

 650° 0. 



(5) It was inferred from the comparisons of different 

 specimens of wire referred to in (3) (which comparisons were 

 independent of all the various sources of error affecting the 

 air-thermometer, and could not have been in error by so much 

 as a tenth of a degree) that the simple parabolic formula did 

 not in all cases represent the small residual differences between 

 the wires. 



(6) It was shown by the direct comparison of other typical 

 metals and alloys with platinum, that the temperature-variation 

 of the resistance of metals and alloys in general could probably 

 be represented by the same type of formula over a consider- 

 able range with nearly the same order of accuracy as in the 



