Measuring the Absolute: William Thomson and the Operationalization of Temperature

Abstract

William Thomson, better known to posterity as Lord Kelvin, made pioneering efforts to create the concept known to us as "absolute temperature", counted in "degrees Kelvin" in his honor. Thomson's work laid the foundation of the thermodynamic concept of temperature established in the latter half of the 19th century, which still provides the theoretical underpinning for the most rigorous measurements of temperature. But it was no easy task to measure absolute temperature in concrete empirical situations. Thomson's definition of absolute temperature was intentionally and resolutely abstract, making no reference whatsoever to any properties of any particular material substances. How was such a concept susceptible at all to measurement by physical instruments? In this paper we examine how absolute temperature was rendered measurable. The results of our investigation should also provide some instructive insights for the general problem of linking abstract theoretical schemes to concrete empirical observations.