Fox's work was in what today would be referred to as geophysics. He was distinguished for his researches on the internal temperature of the earth, contributing papers to the Royal Geological Society of Cornwall, and being the first to prove that temperature definitely increases with depth (the geothermal gradient), his observations being conducted in Cornish mines from 1815 for a period of forty years. In 1829 he began a set of experiments on the artificial production of miniature veins by means of the long-continued influence of electric currents, and his main results were published in 1836.