Torsional pendulum

When a hanging wire (or quartz fiber) is twisted the restoring force is proportional to the angle q and to the fourth power of the radius. A thin fiber provides a weak restoring force and simple harmonic oscillation of very long period.

Torsional pendulums are useful as null instruments to measure small forces. The electrostatic attraction between small charged spheres (Coulomb) and the gravitational attraction between uniform spherical masses (Cavendish).

Measuring G is a very delicate operation with very small deflections.

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