GEOPHYSICAL LABORATORY. lOI 



Xo effort has been made to prepare metals of exceptional purity in our 

 own laboratory, for the reason that such metals would not be available for 

 general use and would therefore be of little service. We have accordingly 

 adopted metals which are carried permanently in stock by dealers whose 

 names are given in connection with each, from whom the same metal in a 

 nominal quality equal to that which we used can be readily obtained. We 

 have analyzed these with extreme care to show the exact content of the sam- 

 ple supplied us. We have duplicated the purchases ourselves, and have found 

 no errors greater than i° in their melting-point determination. 



Another difficulty arises from the fact that the melting-points of the purest 

 metals available for use as constants in reproducing a high-temperature scale 

 Czinc, silver, gold, copper, and palladium) are distributed in such a way that, 

 although they may be located upon the gas-thermometer scale with a proba- 

 ble error not greater than 0.5° at low temperatures or 1° at high tempera- 

 tures, the calculation of a similar curve passing through these points may not 

 suffice to reproduce the scale with this accuracy. In the region midway 

 between zinc (418.2°) and silver (960.0°) the error of interpolation may 

 amount to 2°, and between copper and palladium to 5°, even with metals of 

 exceptional purity. Extrapolation is even more uncertain. 



In order to facilitate as far as possible the application of these results in 

 general practice, a typical thermo-element curve has been tabulated in small 

 10° intervals throughout its entire length from melting ice to melting plati- 

 num, together with a diagram showing the character and magnitude of the 

 variation from this curve which may be expected to appear in other thermo- 

 elements of the same nominal composition (90 parts platinum, 10 parts rho- 

 dium). With a new platin rhodium thermo-element of undoubted homoge- 

 neity, but unknown constants, it is quite practicable with this table to prepare 

 a curve of its electromotive force for any temperature with sufficient accu- 

 racy for most purposes (say 5° at low temperatures and ± 10° above 1200°) 

 from a single determination in melting copper. If this accuracy is inade- 

 quate, additional determinations of its electromotive force at other tempera- 

 tures of the list below will help to fix it more closely. 



There is no sure way to guard against the contaminating influence of 

 metal-vapors upon a thermo-element in laboratory or industrial practice, al- 

 though glazed porcelain is usually effective. There are very simple and rapid 

 means of detecting contamination in an element and determining its distribu- 

 tion, and with a second element at hand for an occasional comparison there 

 is little of serious danger from this cause. In any case, the slight inconven- 

 ience is well worth while wherever considerable accuracy is sought, for there 

 is no other device yet available, in the region between 1100° and 1600°, 

 which is comparable with the thermo-element in sensitiveness and general 

 practicability. 



In conclusion, the list of standard melting-points is given in tabular form, 

 together with an estimate of the degree of trustworthiness to be accorded to 

 each. Beside it for convenient comparison is the present Reichsanstalt scale. 

 It may be added that no indication of a limit to the temperature attainable 

 with the nitrogen thermometer or to its ultimate accuracy was discovered 

 during the present investigation. 



