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Torsion Balance – 1785

Charles-Augustin de Coulomb didn't invent the torsion balance, but he was the first to discover it could be used to measure electrical charge – the first device capable of such a feat.

Torsion Balance – 1785

In early investigations of electricity, scientists had few tools to aid them. By the 1780s, devices to generate, store and detect static electricity had been built, but there was no easy way to gauge amounts of static electrical charges. A French engineer with an interest in electricity and magnetism, Charles-Augustin de Coulomb, developed one of the earliest instruments capable of this feat: the torsion balance.

Coulomb’s torsion balance consists of several small parts, pictured at right. Within a glass case, which prevents breezes or other environmental factors from affecting results, a needle hangs from a thread, typically of silk. A narrow glass tube extends through the top of the glass case. At the top of the tube is a metal sphere, from which the needle hangs by the thread. A small metal sphere is at one end of the needle, which can swing freely due to its suspended state. Protruding through the top of the glass case is also a metal rod with metal spheres at both ends (one inside the case, one outside).

To use the torsion balance, Coulomb would hold an object near the metal sphere at the upper end of the metal rod. Any charge held by the object being studied would transfer to the metal sphere, then travel along the rod to the sphere at the other end. There the charge could affect the needle suspended in the case, which in its resting state touched the rod's lower sphere. So any charge in that sphere passed to the needle’s sphere. Once the rod’s sphere and the needle’s sphere both became similarly charged, they repelled one another. The repulsion caused the needle to move and the thread holding it to twist. The twisting action is called torsion, hence the name of the instrument. To determine how much torsion occurred, Coulomb consulted a small scale marked in degrees near the upper end of the narrow glass tube. A second scale encircling the glass case itself allowed him to determine how far the needle moved. As Coulomb realized, the greater the charge, the greater the torsion and displacement he observed.

An earlier scientist, John Michell, had used a similar instrument to study gravity, but the device did not gain much fame until after Coulomb reinvented it and put it to a different use. Coulomb carried out detailed studies of electrostatic forces with the torsion balance that allowed him to offer the world proof of the inverse square law that today bears his name. According to Coulomb’s law, the electric force between objects is inversely proportional to the distance between the objects. A similar inverse square law exists for gravity, but gravitation is influenced by the masses of objects rather than their charges.

After Coulomb published the results of his investigations and a description of the torsion balance, scientists around the globe wanted the tool. In fact, the torsion balance became one of the most popular scientific instruments to grace laboratories in the late 18th century and throughout the following century.