Instruments
Torsion Balance
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The torsion balance, also called the torsion pendulum, is a scientific apparatus for measuring very weak forces from the twist it produces in a thin suspended fiber. It is usually credited to Charles-Augustin de Coulomb, who used it to establish the inverse-square law of electrostatic force, though John Michell built one independently before 1783. Michell's instrument passed after his death to Henry Cavendish, who modified it and used it in 1797 and 1798 to measure the gravitational attraction between lead spheres, yielding the first accurate value for the mean density of the Earth and, later, the gravitational constant.
Facts
Invented YearSourced to the subject's own accountCommonly credited to Coulomb for this year; John Michell built one independently before 1783, and it was Michell's instrument, inherited and modified by Henry Cavendish, that measured the Earth's density in 1797-98. MeasuresSourced to the subject's own accountVery weak forces (electrostatic force, gravitational attraction), by the angle a suspended fiber twists under them 1 Operating PrincipleSourced to the subject's own accountA horizontal bar hangs from a thin fiber or wire. An applied force turns the bar, twisting the fiber; the twist angle, calibrated against the fiber's known torsion constant, gives the size of the force. 1 ResolutionSourced to the subject's own accountIn the historic Cavendish experiment, the torsion balance's angular deflection could be measured to a precision better than 0.01 inches (0.25 millimeters) using vernier scales on the ends of the rod. 2 Operating RangeSourced to the subject's own accountIn the Cavendish experiment, the torsion balance deflected by about 0.16 degrees with the standard wire, or about 0.03 degrees with a stiffer wire, in response to a twisting force of about 1.74 times ten to the minus seven newtons, the weight of roughly 0.0177 milligrams. 2 Cross-Tradition Connections
Associated With
Cavendish did not invent the torsion balance; he received John Michell's instrument after Michell's death, modified it, and used it in 1797-98 to measure the Earth's density.
Invented By
Built independently before 1783; Michell died before completing an experiment with it, and his instrument passed to Henry Cavendish.
Used In
Michell's torsion balance, inherited and modified by Henry Cavendish, is the instrument the Cavendish experiment is built around.
Sources
1. Torsion Balance (Wikipedia)
WikipediaTorsion balance sectionQuote, Torsion balance section
The torsion balance, also called torsion pendulum, is a scientific apparatus for measuring very weak forces, usually credited to Charles-Augustin de Coulomb, who invented it in 1777, but independently invented by John Michell sometime before 1783.
2. Cavendish Experiment (Wikipedia)
WikipediaThe experiment section (vernier-scale precision)Quote, The experiment section (vernier-scale precision)
Cavendish was able to measure this small deflection to an accuracy of better than 0.01 inches (0.25 mm) using vernier scales on the ends of the rod.
2. Cavendish Experiment (Wikipedia)
WikipediaThe experiment section (deflection angle)Quote, The experiment section (deflection angle)
The key observable was the deflection of the torsion balance rod, which Cavendish measured to be about 0.16 degrees (or only 0.03 degrees for the stiffer wire used mostly).
John Michell (Wikipedia)
WikipediaInvented By: John Michell, Gravity sectionQuote, Invented By: John Michell, Gravity section
Michell devised a torsion balance for measuring the mass of the Earth, but died before he could use it. His instrument passed into the hands of his lifelong friend Henry Cavendish, who first performed in 1798 the experiment now known as the Cavendish experiment.
Henry Cavendish (Wikipedia)
WikipediaAssociated With: Henry Cavendish, Density of the Earth sectionQuote, Associated With: Henry Cavendish, Density of the Earth section
The apparatus Cavendish used for weighing the Earth was a modification of the torsion balance built by geologist John Michell, who died before he could begin the experiment. The apparatus was sent in crates to Cavendish, who completed the experiment in 1797-1798.
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