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Extensions of Callendar's Equations for Platinum Resistance Thermometry

### Abstract

Callendar's equations are generalized to permit interpolation between arbitrary fixed points. Fixed points appropriate to a desired temperature region may thus be employed, rather than Callendar's fixed 0 and 100°C calibration points. This should afford greater accuracy, especially in industrial measurements where the platinumthermometer and the measurement means may be less than ideal. To this end, Callendar's definition of *platinum temperature* is generalized to mean the temperature found by linear interpolation with a platinumthermometer between given fixed points and using a given measurement method. The true temperature may then be found either by recursion or by an extension of Callendar's correction scheme. It is shown that the effect of a general calibration interval *T* _{1}, *T* _{2}, rather than Callendar's 0 to 100°C interval, is to alter the effective value of δ in a calculable way. When deflection rather than balanced‐bridge methods are used, it is shown that the effect of bridge nonlinearity may be approximated by an additional calculable alteration of δ.

© 1969 The American Institute of Physics

Received 07 July 1969
Revised 31 July 1969
Published online 10 November 2003