Replication of Coulomb's Torsion Balance Experiment

Abstract

A long-standing issue in history of science is whether or not the fundamental law of electrostatics was justified experimentally in the late 1700s. By that time, following Isaac Newton’s mechanics, physicists had come to understand that the gravitational force between two masses m1 and m2 may be expressed by Fg ∝ m1m2 r2 , where r is the distance between the centers of the two bodies. The success of this inversesquare law led some individuals to investigate whether phenomena other than celestial motions could also be described by inverse-square equations. In particular, some scientists, Joseph Priestly and Henry Cavendish among them, argued that the repulsion and attraction between bodies charged by electricity are likewise described by an expression such as Fe ∝ q1q2 d2 , where each q represents the particular quantity of electric charge on each body, and d is the distance between their centers. The remarkable formal coincidence between such force laws prompts the question: had scientists found substantive evidence to warrant that law of electrostatics or did they mainly presuppose that it matched Newton’s law? In June of 1785, Charles Augustin Coulomb, a retired military engineer, announced to the Paris Academy of Sciences that he had devised an innovative experimental apparatus, the torsion balance, an extremely sensitive instrument able to measure even minute forces to an unprecedented degree of accuracy. With it Coulomb claimed to have demonstrated that electrostatic repulsion indeed varies inversely at the square of the distance. That experiment, together with another presented in 1787, eventually led physicists to designate the fundamental equation of electrostatics as “Coulomb’s law.” Coulomb was admired increasingly as having helped to transform French physics from a descriptive field (plagued by dubitable speculative hypotheses) to a conceptually lean, experimentally grounded, and highly mathematized science.1 His rigorous engineering mindset