234 PROCEEDINGS OP THE AMERICAN ACADEMY. 



Ikvestigations on Light and Heat, made and pcblished wholly ob in pabi with 

 Appkoprlation from the Rumford Fund. 



XII. 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF 

 THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



XLVI. — PYROMETRY : CALIBRATION OF THE 



LE CHATELIER THERMO-ELECTRIC 



PYROMETER. 



By Silas W. IIolmax. 



Presented November 13, 1895. 



In a discussion of thermo-electric interpolation formuLe * the 

 author has shown that the resultant thermal emf. S^e of a closed 

 metallic circuit of two metals, with one junction at 0° C. and the other 

 at t° C, could be expressed to 0.5 per cent or better above 300° C. by an 

 expression of the form ^'^e — m f or log SoC = n log t + log m, where 

 m and n are constants depending on the metals of the couple. Similar 

 demonstrations show that with the cold junction constantly at 20° C. 

 the emf. may be expressed by 2^0^ = m t" nearly enough. This fact, 

 together with the known fact that the D'Arsonval galvanometer gives 

 readings nearly proportional to the currents, and therefore to 2 e on 

 a circuit of constant resistance, led to the thought that the calibration 

 of the Le Chatelier thermo-electric pyrometer might be simplified by 

 the use of a logarithmic instead of a direct plot. A study of a series 

 of six calibrations made at different times by four different sets of 

 observers, with two distinct pyrometers, and in nearly every case 

 with different suspension wires and adjustment, confirmed the deduc- 

 tion and led to the following method. It will be seen that the method 

 requires calibration at only two known temperatures, instead of several, 

 as formerly, — a saving of labor and a gain in accuracy. The method, 

 although suggested by the facts referred to, is not merely a deduction 

 from them. It should rather be regarded as an empirical process 

 based on experiment. It is not rigidly exact, viewed from either a 

 mathematical or experimental standpoint, but is merely an approxi- 



* Thermo-electric Interpolation Formulffi, ante, p. 193. 



