Cavendish experiment

In 1798, Henry Cavendish published an account of experiments intended to determine Earth's density. The underlying apparatus had been devised by the geologist John Michell, who died before conducting the measurements; Cavendish adapted and used it. In a time when Earth's mass could not be weighed directly, comparing the attraction between laboratory masses with Earth's pull offered a way to estimate the planet's mean density.

Cavendish suspended a light, horizontal wooden arm from a thin wire, with a small lead sphere at each end. Large lead spheres were placed near the smaller ones. Their gravitational attraction exerted a tiny torque on the arm, twisting the wire. Cavendish inferred the force from the arm's deflection and oscillations, while taking precautions to shield the delicate apparatus from air currents and other disturbances. The method made the extremely weak gravitational interaction between ordinary objects measurable in a laboratory.

Cavendish reported Earth's density as 5.48 times that of water. His paper framed the result as a determination of Earth's density, not as an explicit measurement of the gravitational constant G. In modern physics, the same basic experimental arrangement can be analyzed to determine G from the attraction between known masses. The distinction matters: Cavendish's original goal and reported quantity were Earth's density, while the present-day interpretation highlights the universal gravitational constant.

The experiment became a landmark in precision measurement and in the experimental study of gravity. Modern Cavendish-type experiments use improved torsion balances, optical readouts and environmental controls to measure the very small gravitational force between laboratory masses. The name is also widely used in physics education for demonstrations or experiments that determine G using this principle.