148 A. L. Day and R. B. Sosman — 



employed almost universally for the determination of tempera- 

 tures above 1100°, not only for direct determinations of tem- 

 perature with the thermoelement itself, but also for the cali- 

 bration of optical pyrometric apparatus. The absence of 

 absolute determinations in this region has left this practice in 

 undisturbed security until recently, when some doubt has been 

 thrown upon the validity of irresponsible upward extrapola- 

 tion by various observations. (1) The increase in the accuracy 

 now attainable with the optical pyrometer has given an inde- 

 pendent thermal scale comparable with that of the thermoele- 

 ment and overlapping the same region. The two curves have 

 not been found to correspond. (2) Experimental determina- 

 tions of the melting point of platinum by continuing observa- 

 tions of the thermoelement up to a point where a portion of 

 its platinum wire melts, have been undertaken in the national 

 laboratories of Germany, England and the United States, and 

 have yielded a value measured upon the extrapolated thermo- 

 electric curve of about 1710°. The agreement in the different 

 determinations was good and the result found general accept- 

 ance for a time. More recently, Holborn and Valentiner have 

 made successful measurements with the gas thermometer at the 

 temperature of melting palladium, and although high accuracy 

 was not attempted, it became clear that the palladium point 

 obtained by extrapolating w T ith the thermoelement was much 

 too low and by inference the platinum point also, for the vari- 

 ous optical methods give opportunity for a very good determi- 

 nation of the temperature difference between the melting 

 points of the two metals. The most recent estimates of the 

 platinum melting point obtained in this way place it between 

 1750° and 1775°, indicating that the upward extrapolation 

 with the thermoelement has given rise to an error of about 

 50° at the platinum point. 



The data obtained in the present investigation throw much 

 light upon this situation. If we take the observations of our 

 series over the range covered by the Reichsanstalt scale (300° to 

 1100°) and write an equation for these of the same type as that 

 used at the Reichsanstalt, it will read, 



E= — 302 + 8-2356*+ '0016393* 2 



and this equation will reproduce the temperatures of the stand- 

 ard melting points which fall in this region with a maximum 

 error of 3 microvolts, an accuracy far within the errors of obser- 

 vation. But if we extrapolate this curve in accordance with 

 the general practice above described, and compare the resulting 

 electromotive forces with our observations between 1100° and 



