Unit 1 Engineering Physics

1.7. TORSION PENDULUM

Figure 1.19: A torsion pendulum

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).

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Master

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

1.7.1 Torsion Pendulum - Determination of Rigidity Modulus

of Suspension Wire

Consider the suspension fibre of a torsion pendulum as a thin cylinder of lengthland
radiusrfixed at the top end and twisted at the free end by an angle◊as shown in Figure
5.65. Due to the elasticity of the wire, an internal restoring force is developed in the wire
and it is said to be in torsion.

PH8151 21 LICET