Apparatus for determining the gravitional constant

图注： Observations of the motions of the planets around the Sun, or the Moon around the Earth, could not yield the gravitational constant, as in those cases, the mass of the central body is not initially known. Measuring the gravitational constant required a situation where the masses involved had been determined independent of measuring their gravitational attraction. The device shown here was built by John Michell (1724–1793), but Michell died before he could perform the experiment. Henry Cavendish (1731–1810) inherited the device, modified it so as to suppress external disturbances, and successfully completed the experiment. Cavendish's report to the Royal Society was under the title of "Experiments to determine the density of the Earth" as, from knowledge of the gravitational constant, the gravitational acceleration at Earth's surface and the Earth's radius, one can determine the Earth's mass and its mean density. From the modern perspective, what is now known as the "Cavendish experiment" is seen as a way of determining Newton's gravitational constant G. The image shows a cross-section of the apparatus, which Cavendish had further isolated from environmental influences by putting it into a separate room and inside a wooden box. The devices allowing Cavendish to illuminate, observe and manipulate the experiment from the outside are pictured as well. The core of the experiment is a torsion balance using two small lead spheres. The restoring force of the torsion pendulum is deduced from its natural oscillation frequency in the absence of the large masses. The gravitational attraction of the small lead spheres to their larger counterparts can then be determined by measuring how far it makes the torsion pendulum masses deviate from their null position. The image is a slightly modified (cropped, contrast and brightness adjusted) version of Fig. 1 in Cavendish's article in the Philosophical Transactions of the Royal Society, Volume 88 (December 1798), pp. 469–526 [DOI: 10.1098/rstl.1798.0022]. The permission of the Royal Society to publish this image under a CC BY licenses gratefully acknowledged.
来源： Henry Cavendish in Philosophical Transactions of the Royal Society, DOI: 10.1098/rstl.1798.0022 来源链接

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