How did William Thomson try to unify ideas about heat, electricity, magnetism, and motion?

William Thomson, Baron Kelvin, sought to unify the disparate fields of physics—including heat, electricity, magnetism, and mechanics—by viewing all physical change as energy-related phenomena. His approach was guided by the conviction that various theories of matter and energy were converging toward a single, unified theory.12

Thomson’s efforts to unify these ideas relied on several key methods:

• Mathematical Analogies: He pioneered the use of mathematical analogies to link different areas of study. Notably, he demonstrated that the equations describing the flow of heat in solid bodies were analogous to those governing the flow of electricity through conductors. He also established mathematical relationships between hydrodynamic phenomena and electric currents.123
• Energy Synthesis: By building on the work of scientists like Jean-Baptiste-Joseph Fourier and George Green, Thomson helped develop the dynamical theory of energy. His endorsement of James Prescott Joule’s theory regarding the interconvertibility of heat and motion was a critical step toward the second law of thermodynamics, which served as a foundation for his unified worldview.12
• Worldview: Thomson believed that forces such as electricity, magnetism, and heat were the result of invisible material in motion, rather than imponderable fluids. This consistent perspective allowed him to apply basic physical principles across multiple disciplines, ultimately influencing James Clerk Maxwell in the development of electromagnetic theory.12