To most students today the mechanical equivalent of heat, called the Mayer-Joule principle, is simply a way to convert from calories to joules and vice versa. However, in linking work and heat—once thought to be disjointed concepts—it goes far beyond unit conversion. Heat had eluded understanding for two centuries after Galileo Galilei constructed an early thermometer. Independently, Julius Robert Mayer and James Prescott Joule found the connection between heat and work, the Mayer-Joule principle.
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Thermal state, as used here, differs from a thermodynamic state, which typically requires two or more variables—e.g., temperature and pressure—for a complete specification.
On a personal note, one of us (Newburgh) did this experiment in high school in 1942. It remained as one of his most satisfying scientific experiences.
This holds for an ideal gas whose internal energy is a function of temperature but not volume. For gases with intermolecular interactions, the internal energy is generally volume dependent. Then the difference between the constant-pressure and constant-volume heating energies is not simply the work done for the constant pressure process. Specifically Mayer's assertion holds only if the difference between the derivatives of internal energy is zero, which is not so for non-ideal gases in general.