How did James Prescott Joule’s experiments change William Thomson’s thinking about heat?

When William Thomson first encountered James Prescott Joule’s theory regarding the interconvertibility of heat and motion in 1847, it challenged the prevailing scientific consensus that heat was an "imponderable substance" known as caloric.1

Although Thomson initially could not accept Joule’s claim that heat was a form of motion, he remained open-minded and reserved judgment, noting that the relationship between heat and mechanical motion aligned with his own views on the causes of force. Over time, Thomson’s thinking shifted as he synthesized these ideas into his broader work on energy. By 1851, he provided public recognition and a cautious endorsement of Joule’s theory in his treatise, "On the Dynamical Theory of Heat," which also introduced his version of the second law of thermodynamics.1

This collaboration eventually deepened; in 1852, Thomson and Joule discovered the Joule-Thomson effect, which demonstrated that the temperature of a gas falls when it expands without performing external work, further solidifying their contributions to the development of thermodynamics.2