‘Weighing’ the world


‘Weighing’ the world

The book of science

Tom Sharp

John Michell, Henry Cavendish geodesy

‘Weighing’ the world

Torsion balance

Sequence of results

Henry Cavendish is given credit for being the first to determine the density of the earth, from which one may derive the gravitational constant, G , which we use to express Newton’s law today:

F = G m1 m2 / r2

where F is the force between two masses, m1 and m2 , separated by a distance r .

John Michell was elected a fellow of the Royal Society in the same year as Cavendish, 1760. Cavendish rebuilt Michell’s torsion balance and conducted the experiment, but Michell was the scientist who realized that this device could measure the extremely small forces of attraction between objects smaller than mountains and, therefore, be used to determine the force of gravity. Cavendish’s lead spheres were two inches and twelve inches in diameter.

Michell’s genius was also shown in his work on magnetism, explaining the causes of earthquakes and tsumanis, and, in a letter to Cavendish that was not published until the 1970’s, explaining the effect of gravity on light and existence and nature of dark holes.

Cavendish is also known for the discovery of hydrogen, for his determination that the atmosphere is composed of one part oxygen and four parts nitrogen (leaving only 1/120th of volume for other gases), and numerous discoveries related to electricity that were not known until a century after he died. But Cavendish believed in phlogiston and his writings seem old-fashioned today; his experiments preceded the naming of the elements that he measured: hydrogen, oxygen, nitrogen, argon. Cavendish called hydrogen ‘inflammable air’ and he called oxygen ‘dephlogisticated air.’

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