History of 'temperature': maturation of a measurement concept.

Accounts of how the concept of temperature has evolved typically cast the story as ancillary to the history of the thermometer or the history of the concept of heat. But then, because the history of temperature is not treated as a subject in its own right, modern associations inadvertently get read back into the historical record. This essay attempts to lay down an authoritative record not of what people in the past thought about what we call 'temperature' but of what they thought about what they called 'temperature' (or one of its cognates), from medieval times to today. It is found that invention of the thermometer had little impact on the concept of temperature. Much more significant were Fahrenheit's invention of a reliable instrument and William Thomson's effort to make a degree of temperature a unit of measure. Overlapping definitions of temperature then emerged in the late nineteenth century, and twentieth-century scientific developments forced physicists to reconsider temperature's conceptual boundaries. It turns out that the concept of temperature has evolved through stages that correspond to four increasingly sophisticated types of measurement. Its maturity sheds light on the philosophy of conceptual change.