The torsion balance, also called torsion pendulum, is a scientific apparatus for measuring very weak forces, invented by John Michell sometime before 1783 but more often credited toCharles-Augustin de Coulomb, who independently invented it in 1777. It's most well-known uses were by Coulomb to measure the electrostatic force between charges to establish Coulomb's Law, and byHenry Cavendish in 1798 in the Cavendish experiment to measure the gravitational force between two masses, leading to a value for the gravitational constant.
The torsion balance consists of abar suspended from its middle by a thin fiber. The fiber acts as a very weak torsion spring. If an unknown force is applied at right angles to the ends of the bar, the bar will rotate,twisting the fiber, until it reaches the equilibrium where the twisting force or torque of the fiber balances the applied force. Then the magnitude of the force is proportional to the angle of thebar. The sensitivity of the instrument comes from the weak spring constant of the fiber, so a very weak force causes a large rotation of the bar.
In Coulomb's experiment, the torsionbalance was an insulating rod with a metal ball attached to one end, suspended by a silk thread. The ball was charged with a known charge of static electricity, and a second charged ball of thesame polarity was brought near it. The two charged balls repelled one another, twisting the fiber through a certain angle, which could be read from a scale on the instrument. By knowing howmuch force it took to twist the fiber through a given angle, Coulomb was able to calculate the force between the balls. Determining the force for different charges and different separationsbetween the balls, he showed that the electrostatic force is proportional to the product of the charges and inversely proportional to the distance between them. This is called Coulomb's law.
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