Henry Cavendish's torsion balance, 1798.
© Science Museum / Science & Society Picture Library
Model (scale 1:48). A torsion balance is an instrument for measuring weak forces of attraction between masses, from the amount by which they twist a fine wire. The amount depends on the universal gravitational constant. The torsion balance was devised by Charles Augustin de Coulomb (1736- 1806) in 1785. The English natural philosopher Henry Cavendish (1731-1810) built a torsion balance to measure the gravitational force between two large masses, so that he could make the first calculation of the mass of the earth.