|
The Cavendish experiment, performed in 1797–98 by British scientist Henry Cavendish, was the first experiment to measure the force of gravity between masses in the laboratory〔(Boys 1894 ) p. 355〕 and the first to yield accurate values for the gravitational constant.〔(Encyclopædia Britannica 1910 ) p. 385 'The aim (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 the Earth's density); for instance: There were previous measurements, chiefly by Bouguer (1740) and Maskelyne (1774), but they were very inaccurate ((Poynting 1894 ))((Encyclopædia Britannica 1910 )). 〕 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 specific gravity of the Earth,〔Clotfelter 1987, p. 210〕 or equivalently the mass of the Earth. His experiment gave the first accurate values for these geophysical constants. The experiment was devised sometime before 1783〔(McCormmach & Jungnickel 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.〕 by geologist John Michell,〔(Cavendish 1798 ), p. 59 Cavendish gives full credit to Michell for devising the experiment〕 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 Henry 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.〔Cavendish, H. 'Experiments to determine the Density of the Earth', ''Philosophical Transactions of the Royal Society of London'', (part II) 88 p.469-526 (21 June 1798), reprinted in (Cavendish 1798 )〕 ==The experiment== The apparatus constructed by Cavendish was a torsion balance made of a six-foot (1.8 m) wooden rod suspended from a wire, with a diameter lead sphere attached to each end. Two lead balls were located near the smaller balls, about away, and held in place with a separate suspension system.〔(Cavendish 1798 ), p.59〕 The experiment measured the faint gravitational attraction between the small balls and the larger ones. The two large balls were positioned on alternate sides of the horizontal wooden arm of the balance. Their mutual attraction to the small balls caused the arm to rotate, twisting the wire supporting the arm. The arm stopped rotating when it reached an angle where the twisting force of the wire balanced the combined gravitational force of attraction between the large and small lead spheres. By measuring the angle of the rod and knowing the twisting force (torque) of the wire for a given angle, Cavendish was able to determine the force between the pairs of masses. Since the gravitational force of the Earth on the small ball could be measured directly by weighing it, the ratio of the two forces allowed the density of the earth to be calculated, using Newton's law of gravitation. Cavendish found that the Earth's density was times that of water (due to a simple arithmetic error, found in 1821 by Francis Baily, the erroneous value appears in his paper).〔(Poynting 1894 ), p.45〕 To find the wire's torsion coefficient, the torque exerted by the wire for a given angle of twist, Cavendish timed the natural oscillation period of the balance rod as it rotated slowly clockwise and counterclockwise against the twisting of the wire. The period was about 20 minutes. The torsion coefficient could be calculated from this and the mass and dimensions of the balance. Actually, the rod was never at rest; Cavendish had to measure the deflection angle of the rod while it was oscillating.〔(Cavendish 1798 ), p.64〕 Cavendish's equipment was remarkably sensitive for its time.〔 The force involved in twisting the torsion balance was very small, ,〔(Boys 1894 ) p.357〕 about of the weight of the small balls〔(Cavendish 1798 ) p. 60〕 or roughly the weight of a large grain of sand.〔A 2 mm sand grain weighs ~13 mg. 〕 To prevent air currents and temperature changes from interfering with the measurements, Cavendish placed the entire apparatus in a wooden box about thick, tall, and wide, all in a closed shed on his estate. Through two holes in the walls of the shed, Cavendish used telescopes to observe the movement of the torsion balance's horizontal rod. The motion of the rod was only about .〔(Cavendish 1798 ), p. 99, Result table, (scale graduations = in ≈ 1.3 mm) The total deflection shown in most trials was twice this since he compared the deflection with large balls on opposite sides of the balance beam.〕 Cavendish was able to measure this small deflection to an accuracy of better than one hundredth of an inch using vernier scales on the ends of the rod.〔(Cavendish 1798 ), p.63〕 Cavendish's accuracy was not exceeded until C. V. Boys' experiment in 1895. In time, Michell's torsion balance became the dominant technique for measuring the gravitational constant (''G'') and most contemporary measurements still use variations of it. This is why Cavendish's experiment became ''the'' Cavendish experiment.〔(McCormmach & Jungnickel 1996 ), p.341〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cavendish experiment」の詳細全文を読む スポンサード リンク
|