In the summer of 1914, Erskine Williamson, a 28‐year‐old Scottish mathematical physicist, joined a new laboratory in Washington, DC. Although he had been hired as a research assistant in geology, he began to attack a broad range of problems in physical chemistry, thermodynamics, and heat flow. His efforts soon extended to high‐pressure geophysics, culminating with his derivation of the pressure‐density relation for Earth's interior and the procedure for determining the pressures and densities as a function of depth in the planet. Though his equation is well known as one of the fundamental relations in geophysics, few are familiar with its discoverer, the breadth of his research, and the background behind its development. Williamson died unexpectedly in 1923 at the peak of his productivity, but during his short career he helped lay the foundations for the modern study of planetary interiors.