Experiment measuring the force of gravity (1797–1798)
Cavendish's diagram of his torsion pendulum, seen from above. The pendulum consists of two small spherical lead weights (h, h) hanging from a 6-foot horizontal wooden beam supported in the center by a fine torsion wire. The beam is protected from air currents inside a wooden box (A, A, A, A). The two large weights (W, W) attached to a separate suspension attract the small weights, causing the beam to rotate slightly. The rotation is read off of vernier scales (S) at either end of the rod. The large weights can be rotated to the other side of the torsion beam (w, w), causing the beam to rotate in the opposite direction.
The Cavendish experiment, performed in 1797–1798 by English scientist Henry Cavendish, was the first experiment to measure the force of gravity between masses in the laboratory[1] and the first to yield accurate values for the gravitational constant.[2][3][4] Because of the unit conventions then in use, the gravitational constant does not appear explicitly in Cavendish's work. Instead, the result was originally expressed as the relative density of Earth,[5] or equivalently the mass of Earth. His experiment gave the first accurate values for these geophysical constants.
The experiment was devised sometime before 1783 by geologist John Michell,[6][7] who constructed a torsion balance apparatus for it. However, Michell died in 1793 without completing the work. After his death the apparatus passed to Francis John Hyde Wollaston and then to Cavendish, who rebuilt the apparatus but kept close to Michell's original plan. Cavendish then carried out a series of measurements with the equipment and reported his results in the Philosophical Transactions of the Royal Society in 1798.[8]
^Boys 1894 p. 355
^Poynting 1911, p. 385.
^'The aim [of experiments like Cavendish's] may be regarded either as the determination of the mass of the Earth,...conveniently expressed...as its "mean density", or as the determination of the "gravitation constant", G'. Cavendish's experiment is generally described today as a measurement of G.' (Clotfelter 1987 p. 210).
^Many sources incorrectly state that this was the first measurement of G (or Earth's density); for instance: Feynman, Richard P. (1963). "7. The Theory of Gravitation". mainly mechanics, radiation and heat. The Feynman lectures on physics. Vol. I. Pasadena, California: California Institute of Technology (published 2013). 7–6 Cavendish’s experiment. ISBN 9780465025626. Retrieved December 9, 2013.
There were previous measurements, chiefly by Bouguer (1740) and Maskelyne (1774), but they were very inaccurate (Poynting 1894)(Poynting1911, p. 386).
^Clotfelter 1987, p. 210
^Jungnickel & McCormmach 1996, p. 336: A 1783 letter from Cavendish to Michell contains '...the earliest mention of weighing the world'. Not clear whether 'earliest mention' refers to Cavendish or Michell.
^Cavendish 1798, p. 59 Cavendish gives full credit to Michell for devising the experiment
^Cavendish, H. 'Experiments to determine the Density of the Earth', Philosophical Transactions of the Royal Society of London, (part II) 88 pp. 469–526 (21 June 1798), reprinted in Cavendish 1798
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