PH8151 Engineering Physics Chapter 1

1.7. TORSION PENDULUM

1.7 Torsion pendulum

Torsion pendulum is an oscillator for which the restoring reaction is torsion. It consists
of a disk-like mass suspended from a thin rod or wire (Figure1.19). When the mass

Figure 1.19: A torsion pendulum

is twisted about the axis of the wire, due to its elasticity the wire exerts a torque on
the mass, tending to rotate it back to its original position. If twisted and released, the
mass will rotate back and forth, executing simple harmonic motion (torsional oscillation
or angular oscillation).

Torsion pendulum was invented by Charles-Augustin de Coulomb (in 1777) who then
used it to measure the electrostatic force between charges to establish Coulomb’s Law.
Later, Henry Cavendish (n 1798) used it in the Cavendish experiment to measure the
gravitational force between two masses to calculate the density of the Earth, leading
later to a value for the gravitational constant [ 4 ]. Torsion pendulum is an extremely
sensitive instrument used in a varied range of scientific and engineering applications. For
example, torsion pendula are often used for time-keeping purposes. The balance wheel in
a mechanical wristwatch is a torsion pendulum in which the restoring torque is provided
by a coiled spring (Figure1.20).

JAEGER-LECOULTRE

Master

Figure 1.20: An example of manual-winding mechanical watch[ 5 ]

PH8151 21 LICET