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A torsion balance measures small forces like gravitational or magnetic forces between masses or charged spheres. It consists of a horizontal rod with spheres at both ends that rotate on a wire. The amount of twist in the wire can be converted into the amount of force between the stationary and moving spheres. The balance can calculate a gravitational constant and Coulomb force constant. Later balances became more precise and can measure infinitesimal forces.
A torsion balance is a device invented to measure very small forces such as gravitational forces between small masses or magnetic forces between charged spheres. The torsion balance consists of a horizontal rod with an identical sphere at both ends. The horizontal rod rotates on a wire which supports it in the centre. Small stationary balls of identical mass, or charges, are placed near the balls at the ends of the rotating rod, attracting them and causing the wire to twist. The amount of twist in the wire can then be mathematically converted into the amount of force between the stationary balls and those on the moving rod.
When the masses of the spheres in a torsion balance are known, then scientists can calculate a gravitational constant to plug into Newton’s inverse square law of gravitation. Small forces between spheres of unknown mass can be derived from these results. The forces between stationary spheres of unknown mass and moving spheres of known mass are found by observing the number of times the horizontal rod swings back and forth in a given time interval. The frequency of the back and forth motion of the rod is related to the torsional stress in the wire, from which the unknown forces can be calculated.
In 1783, a physicist, Charles-Augustin de Coulomb, published his discovery that the inverse square law, first proposed by Newton to describe gravitational forces, could be applied to attractive or repulsive magnetic charges. In Coulomb’s law, the attractive or repulsive forces between objects, due to their magnetic characteristics, require a constant, the Coulomb force constant. When the charges on the movable and stationary spheres of the torsion balance are known, the constant can be calculated. Next, stationary spheres of unknown charges could be installed, and the attractive or repulsive forces between them and the moving spheres could be calculated by measuring the frequency of the horizontal rod’s back and forth motion.
Later torsion balances became more sophisticated and precise in their measurements. Scientists noticed that by giving an initial push to the horizontal rod of the balance, the extremely small resistance of the metal atoms in the thin wire supporting the rod could cause it to rotate horizontally back and forth at a certain speed. The relationship between the torsional stresses in the wire when exposed to the infinitesimal forces between spherical bodies continues to successfully measure the unknowns in the inverse square law equations.
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