June 29, 2016

Pendulum of torsion

Vertical pendulum versus torsion pendulum (1890)

The physicist Eötvös developed a new type of pendulum which quickly supplanted the vertical pendulum.

Between 1890 and 1920, as the vertical pendulum method showed its limits, an innovation allowed to go much further–the torsion pendulum. Thanks to this device, the British physicist Cavendish measured the gravitational constant (G) in 1798.

To test the equivalence principle, the idea of the Hungarian physicist Eötvös was to suspend two masses of different composition at both ends of a torsion balance beam. Measurement consists in checking that the beam turns by 180° when the head of the wire is turned by the same quantity. Since the masses undergo the terrestrial field of gravity–which acts on the gravitational mass–and centripetal acceleration related to rotation of the Earth–which acts on the inertial mass–a difference would be recorded if the inertial and gravitational masses were not rigorously equal. The absence of this difference, measured by Eötvös, validated the equivalence principle to an accuracy of 10-8.

This experiment aims at highlighting compensation of the inertial and gravitational forces on motionless masses. Because of the accuracy of measurements, experimental noises are at the same level as the fundamental physics fluctuations.
These noises limit the precision of the experiment in the later versions of the torsion balance.

Balance de torsion
Torsion balance invented by Eötvös.
It also was used too to measure the gradients of the field of gravity.