The freezing point of platinum

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The International Temperature Scale, which has been in force since 1927, is based on certain values assigned to the boiling and freezing points of pure substances and on specified means of interpolation between, or extrapolation beyond, these points. The highest basic point of the scale is the freezing point of gold, defined as 1063·0° C, while for extrapolation from this temperature use is made of the Wien law of radiation, with a certain value of the constant C 2 . Though any temperature above 1063° C is thus completely defined without reference to further fixed points, determinations of such points are of considerable value. In particular, they serve to indicate the degree of reproducibility of the scale by the various users of it, and, when well authenticated, to provide secondary standards for its realization. Of such fixed points the most important has been the freezing point of palladium (1555° C), but the latest developments in furnace technique and refractory materials should now enable the freezing point of platinum to be used with equal, if not greater, advantage. The qualities of platinum which render it especially valuable in this connection are as follows: its freedom from oxidation; its high standard of purity, for which a convenient electrical test is available; its high freezing point (about 1775° C) which approaches the important zone of temperature covered by the electric fighting industry These qualities also make the platinum point especially suitable as the basis for a standard of fight, as has been proposed by a number of experimenters. It is with the two objects indicated above that the National Physical Laboratory has undertaken an investigation concerning the freezing point of platinum the precise scope of which may be defined as follows:— (1) To determine the value of the freezing point in terms of the International Temperature Scale.