Cavendish Measures the Density of Earth

In 1797 and 1798, the English natural philosopher Henry Cavendish conducted a delicate laboratory investigation that yielded the first accurate measurement of the density of Earth. The core concept and initial hardware originated with the English geologist John Michell, who had constructed a torsion balance before 1783. Michell died in 1793 before he could finish his planned trials, leaving the apparatus to Francis John Hyde Wollaston. The equipment subsequently passed to Cavendish, who reconstructed the mechanism closely according to Michell's design before initiating an exhaustive series of observations.

The reconstructed balance centered on a horizontal wooden rod six feet long, suspended by a wire. At each end of this rod hung a lead sphere measuring two inches across and weighing 1.61 pounds. Outside this suspended system, two massive lead spheres, each one foot in diameter and weighing 348 pounds, swung on separate supports. When Cavendish brought these large masses within 8.85 inches of the smaller spheres, their gravitational attraction exerted a slight twisting force on the suspension wire, pulling the horizontal beam toward them.