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A torsion balance is a device invented to measure very small forces like gravitational forces between small masses or magnetic forces between charged spheres. The torsion balance consists of a horizontal rod with an identical sphere on either end. The horizontal rod rotates on a wire that supports it at its center. Fixed, small spheres of identical masses, or charges, are placed close to the spheres 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 spheres 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 insert in Newton’s inverse square law of gravitation. From these results, small forces between spheres of unknown mass can be derived. The forces between stationary spheres of unknown mass and the moving spheres of known mass are found by observing the number of times the horizontal rod swings back and forth in a given amount of time. The frequency of the rod’s back and forth movement 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, attractive or repulsive forces between objects, due to their magnetic characteristics, required a constant, the Coulomb force constant. When the charges on the moving and fixed spheres of the torsion balance are known, then the constant can be calculated. Subsequently, fixed spheres of unknown charges could be installed and attractive or repulsive forces between them and the moving spheres can be calculated by measuring the frequency of back and forth motion of the horizontal rod.
Successive torsion balances have become more sophisticated and precise in their measurements. Scientists noted that by giving the horizontal rod in the balance an initial push, the extremely small resistance of the metallic atoms in the thin wire supporting the rod might cause it to rotate horizontally back and forth at a certain rate. The relationship between the torsional stresses in the metal wire when exposed to the infinitesimal forces between spherical bodies continues to measure successfully the unknowns in inverse square law equations.