Torsion balance

A torsion balance measures small forces or torques through the angular twist of a suspended fiber. Its sensitivity comes from the fiber’s small resistance to twisting.

Torsion pendulum: An oscillator whose restoring force comes from the twist of a suspended fiber or wire. The balance’s suspended mass and fiber form a torsional oscillator.
Cavendish experiment: An eighteenth-century experiment that measured the gravitational attraction between laboratory masses. Henry Cavendish used a torsion balance to determine Earth's mean density.
Gravitational constant measurement: Experimental determination of the constant that sets the strength of gravitational interaction. Torsion balances can measure gravitational attraction between nearby masses.
Torque: The rotational effect of a force, determined by its magnitude and lever arm. The balance responds to applied torque as well as to force.
Spring scale: A device that estimates force from the extension or compression of a spring. It detects linear deformation, whereas a torsion balance detects angular twist.
Torsional stiffness: A measure of the torque required to produce a unit of angular twist. It sets how much the fiber turns under a given force.
Coulomb's torsion balance: An instrument used by Charles-Augustin de Coulomb to study forces between electrically charged bodies. Coulomb used fiber twist to quantify electrostatic force.
Equivalence principle: The principle that inertial and gravitational responses are equivalent under specified conditions. A balance can detect composition-dependent accelerations that would violate it.
Moment of inertia: A measure of how strongly a body resists angular acceleration about an axis. It helps determine the balance’s oscillation period.
Force transducer: A device that converts an applied force into a measurable signal. Torsion balances are one specialized kind of force-measuring instrument.

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