Torsion Balance to Measure G

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© Blayne Heckel.

The diagram above shows the layout of a torsion balance measurement of the universal gravitational constant, G. A thin, 20 µm diameter fiber supports two test masses (red) and a mirror that reflects laser light to a position-sensitive detector to measure the twist angle of the pendulum. Two massive spheres (black) are moved in a circular path around the pendulum and exert changing gravitational forces on the pendulum, causing it to twist. By measuring the twist angle, masses, distances, and conversion factor of twist-per-unit torque, the gravitational constant can be deduced. (Unit: 3)