How did the torsion balance in the Cavendish experiment work?

The torsion balance used in the Cavendish experiment was an apparatus designed to measure the incredibly weak gravitational attraction between masses.12

The device consisted of a wooden rod suspended freely from a thin wire, with a small lead sphere attached to each end. Two much larger lead spheres were positioned near these smaller ones. The gravitational attraction between the large and small spheres caused the rod to rotate, twisting the supporting wire. This movement was counteracted by the wire's natural restoring force, causing the rod to oscillate like a horizontal pendulum.12

To ensure extreme precision, Henry Cavendish placed the apparatus in a sealed room to eliminate external disturbances, such as air currents or temperature fluctuations. He observed the rod's motion from outside using a telescope. By measuring the distance the rod moved and the period of its oscillation, Cavendish calculated the gravitational force between the spheres. He then used this data to determine the density of the Earth, achieving a result within 1 percent of the modern value.12