114 



A. L. Day and R. B. Sosman — 



a positive check upon the appearance of contamination in the 

 thermoelements.* 



With a half meter bar and a temperature interval extending 

 from zero to 1400°, the total expansion amounts to about 

 7*8 mm . The micrometers reading the expansion were read 

 with an accuracy of 0*002 mm . 



There was some indication of a very small hysteresis in the 

 expansion and contraction. Although the amount was not 

 much greater than the experimental error, the measurements 

 indicate that the bar was slightly shorter after heating than 

 before, and that it gradually regained its original length. 



The measurements at room temperature are given in Table 

 II. The five measurements in this table which were made 



Table II. — Length of Platinum- Rhodium Bar. 



Date 



Max. preced- 

 ing temper- 

 ature 



Length. 

 at0° 



Date 



Max. preced- 

 ing temper- 

 ature. 



Length 

 at0° 



1 July 1908 



(New) 



500-068 



26 Sept. 1908 



1150° 



500-094 



6 " " 



900° 



500-110* 



1 Oct. " 



25° 



500-119 



9 " 



28° 



500-105 



6 " 



1300° 



500-034f 



13 " 



900° 



500-098* 



i27 " " 



900° 



500-108* 



17 Sept. " 



900° 



500-108* 



30 " " 



1400° 



500-096 



19 " 



1200° 



500-090 



6 " 1909 



28° 



500-103 



20 " 



23° 



500-105 



12 " " 



22° 



500-108* 



22 '■ 



1200° 



500-087 



113 " " 



1000° 



500-109 



24 " 



24° 



500-096 



|15 " " 



1400° 



500-074f 



* After interval of 4-7 days, 

 f Bent, after heating beyond 

 were made. 



last temperature at which measurements 



within a few hours after the bar had cooled from a high tem- 

 perature, excluding the two where the bar was bent, average 

 500*095 ; while the ten measurements (excluding the first) 

 which were made two days or more after heating, average 

 500*106. The difference is only 0*002 per cent of the total 

 length, or 0*12 per cent of the total expansion to 1500°, or 

 about 0*7 per cent of the expansion to 300°. This effect is, 

 therefore, probably responsible for the observed irregularities 

 between 0° and 300°, at which temperature most of the meas- 

 urements were begun. f 



C. Transference of Temperature by the Thermoelement. — 

 The electromotive forces of the elements attached to the bulb 

 were measured by a Wolff potentiometer. The standard of 



*Day and Clement, loc. cit., p. 419. 



\ Kammerlingh-Onnes (Konink. Ak. Wet. Amsterdam, Proc, x, 342, 

 1907) has found the same effect after cooling platinum to very low tempera- 

 tures. 



