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.