Joule, Mayer, and Others: A Decade-and-a-Half-Long Debate over Priority in the Discovery of the Mechanical Equivalent of Heat

This article reviews the history of a debate over priority in the discovery of the mechanical equivalent of heat that was centered around J. P. Joule and J. R. von Mayer. The following two stages may be distinguished in this debate. During the first stage, those involved in it were Joule and Mayer themselves. While Mayer presented a numerical value for the mechanical equivalent of heat, which was based on the data from Gay-Lussac’s experiment, Joule determined the value of this coefficient in his own experiment although he did it later than Mayer (actually, Joule was unaware of Gay-Lussac’s experiment). This article shows that, in the end, Joule and William Thomson, who also participated in the debate, recognized (even though formally and with reservations) Mayer’s priority. During the second stage of the debate, its participants were British scientists who supported Mayer or Joule. Thus, Mayer’s priority was supported by Professor J. Tyndall of the Royal Institution in London and it was he who initiated the resumption of the discussion. Joule’s priority was advocated by Professor W. Thomson of the University of Glasgow and Professor P. Tait of the University of Edinburgh. It is noted that a personal animosity between Tyndall and Tait, as well as Tyndall’s competitive attitude towards Thomson, had a significant impact on the tone of the debate, and the examples of Tait’s provocative remarks and Tyndall’s reactions are provided. Joule’s involvement during the second stage of the debate that was mostly limited to private correspondence between himself, Tait, Thomson, and Tyndall, is discussed. Over the time elapsed after the first stage of the debate, the level of rejection of Mayer’s arguments by the scientific community had decreased significantly. The awarding of the Royal Society’s Copley Medal to Joule (1870) and Mayer (1871), both of them nominated by Tyndall, came as a symbolic conclusion of the debate.