The melting point of palladium

The melting point of palladium is a convenient reference point for the measurement of high temperatures. In fixing a scale of temperature the aim is, of course, to approximate as closely as possible to the thermodynamic scale. From the absolute zero up to moderately high temperatures this ideal scale is realised most directly and accurately through the medium of the gas thermometer. However, with increase of temperature beyond a certain limit, the experimental difficulties of gas thermometry multiply rapidly, so that ultimately it becomes necessary to adopt another basis for obtaining the scale. This is conveniently found in the laws governing the radiation from a black body, which have a sound theoretical foundation and permit the use of measuring instruments of precision. The establishment of a practical scale of temperature on the lines above indicated has been the subject of considerable discussion between the national standardising laboratories of Germany, Great Britain and the United States of America. As a result, proposals for the definition of an "International Temperature Scale" were submitted to the 7th General Conference of Weights and Measures, and approved by them. In effect, the basis of the scale up to the melting point of gold is the gas thermometer, and beyond this temperature the Wien or Planck law of radiation with an agreed value for the constant c 2 . Owing to the difficulty of absolute measurements of radiation, no attempt has so far been made to place the radiation scale on an independent basis by fixing the other constant in the Wien or Planck equation. Consequently the scale is defined, for the present, relatively to a fixed point on the thermodynamic scale, as given by the gas thermometer, namely the melting point of gold (1063°C.).