A sensitive instrument with a beam suspended on a thin wire, used to measure tiny gravitational or inertial forces.A torsion balance is a remarkably simple yet exquisitely sensitive instrument. It consists of a beam suspended from a thin wire or fiber. When a force acts on the beam, it twists the wire, and the amount of twist is a measure of the force. This design allows for the detection of extremely tiny forces, such as the gravitational attraction between small masses in a laboratory. It has been central to some of the most important experiments in physics, from measuring the gravitational constant to testing the equivalence principle.
The torsion balance was invented by John Michell, but it was Henry Cavendish who used it in 1798 to measure the gravitational constant, an experiment now known as the Cavendish experiment. Charles-Augustin de Coulomb also independently used a torsion balance to study electrostatic forces.
The beam is balanced so that the only restoring force is the torsion of the suspension wire. A small force applied at the ends of the beam causes it to rotate until the torque from the wire equals the applied torque. By measuring the angle of rotation, often with a laser and mirror, the force can be determined with incredible precision. The sensitivity is limited by thermal fluctuations in the wire, but careful design minimizes these.
Torsion balances are used in modern physics to test the equivalence principle, to measure the gravitational constant with high precision, and in some sensitive accelerometers.
The Cavendish experiment, using a torsion balance, is often called the 'weighing the Earth' experiment because it allowed the mass of the Earth to be calculated for the first time.
