Maxwell's thermodynamic surface



Maxwell’s thermodynamic surface is an 1874 sculpture made by Scottish physicist James Clerk Maxwell showing the various states of a fictitious water-like substance on a three-dimensional volume-energy-entropy plot, based on the American scientist Willard Gibbs’ graphical thermodynamics papers of the early 1870s.

Gibbs' papers defined what Gibbs called "the thermodynamic surface" in term of the triple volume-energy-entropy. After receiving reprints of Gibbs' papers, Maxwell recognized the insight afforded by Gibbs' new point of view and set about to construct physical three-dimensional models of this surface.

Maxwell sculpted the original model in clay and made three plaster casts of the clay model, sending one to Gibbs as a gift, keeping the other two in his laboratory at Cambridge University. Maxwell's copy is on display at the Cavendish Laboratory of Cambridge University, while Gibbs' copy is on display at the Sloane Physics Laboratory of Yale University, where Gibbs held a professorship.

Maxwell drew lines of equal pressure (isopiestics) and of equal temperature (isothermals) on his plaster cast by placing it in the sunlight, and "tracing the curve when the rays just grazed the surface." One of his letters reports sending sketches of these lines to Thomas Andrews.