Henry Cavendish’s 1798 experiment calculated Earth’s density by measuring the attraction between lead spheres.
Cavendish used a torsion balance: two small lead balls were attached to a light horizontal rod, while two much larger lead spheres were positioned nearby. Their weak gravitational attraction twisted the suspension wire by a tiny, measurable amount. From this deflection, Cavendish determined the strength of gravity and inferred Earth’s average density.
The experiment is often called “weighing the Earth,” although Cavendish did not directly measure Earth’s mass with a scale. His published result was an estimate that Earth was about 5.48 times as dense as water, close to the modern value of about 5.51. The work also provided an early measurement related to the gravitational constant, though Cavendish framed the result mainly as Earth’s density.
The experiment’s importance extended beyond geophysics. It demonstrated that gravity could be measured in a laboratory between ordinary objects, not only observed in planetary motion.