Progress in platinum resistance thermometry

The development of accurate platinum resistance thermometry from the time when it was put on a sound basis by Callendar seventy-five years ago to the present day is described. It is shown how accuracy has been continually increased through improvements in the design of thermometers, methods of measurement and the reproducibility of the thermometric fixed points on which the thermometer calibration is based. The possibility of extending the range of the platinum thermometer as the standard interpolation instrument for the International Practical Scale of Temperature beyond the present limits of - 182.97° C to 630.5° C is dealt with: the problems associated with measurements down to 20° K and up to 1063° C are discussed and some of the practical solutions which have been proposed are described. The relation between the thermodynamic scale and that obtained by interpolation between fixed points with the platinum thermometer between - 182.97° C and 444.6° C is also discussed.