What physical constant did Henry Cavendish first determine using two weights and a cantilever?

Henry Cavendish first determined the gravitational constant using a delicate balance and pairs of lead weights. His experiment, carried out in 1797–1798, measured the extremely small gravitational attraction between masses in the laboratory.

The apparatus was technically a torsion balance rather than a simple cantilever. Two small lead spheres were attached to a horizontal rod suspended by a thin wire, while two much larger lead spheres were positioned nearby. Their attraction twisted the wire, and the tiny deflection revealed the force between the masses.

Cavendish originally described his goal as measuring Earth’s density, often remembered as “weighing the Earth.” By comparing the laboratory attraction with Earth’s pull, he obtained the information needed to calculate Earth’s density and, in modern terms, the universal gravitational constant, G. Cavendish himself did not write down G in its modern form because that notation and framing came later.

A common mix-up is confusing capital G with lowercase g: G is the universal gravitational constant, while g is the local acceleration caused by gravity, about 9.8 m/s² near Earth’s surface.