Von Eotvos's torsion scale. This device, designed to measure gravitational acceleration, was built and operated by the Hungarian physicist Lorand von Eotvos (1848-1919). His 1889 experiment measured the differences in the gravitational acceleration of different masses. This difference was found to be effectively zero, showing that mass and inertia are equivalent. This is a central postulate of Einstein's 1915 theory of General Relativity. A torsion scale can measure tiny attractions between laboratory-scale masses, doing so by small changes in the torsion of twisted wires.