Cavendish experiment

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] 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,[4] 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 by geologist John Michell,[5][6] 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.[7]

The experiment

The apparatus constructed by Cavendish was a torsion balance made of a six-foot (1.8 m) wooden rod horizontally suspended from a wire, with two 2-inch (51 mm) diameter 1.61-pound (0.73 kg) lead spheres, one attached to each end. Two 12-inch (300 mm) 348-pound (158 kg) lead balls were located near the smaller balls, about 9 inches (230 mm) away, and held in place with a separate suspension system.[8] The experiment measured the faint gravitational attraction between the small balls and the larger ones.

Vertical section drawing of Cavendish's torsion balance instrument including the building in which it was housed. The large balls were hung from a frame so they could be rotated into position next to the small balls by a pulley from outside. Figure 1 of Cavendish's paper.
Detail showing torsion balance arm (m), large ball (W), small ball (x), and isolating box (ABCDE).

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 Specific gravity of the Earth to be calculated, using Newton's law of gravitation.

Cavendish found that the Earth's density was 5.448±0.033 times that of water (due to a simple arithmetic error, found in 1821 by Francis Baily, the erroneous value 5.480±0.038 appears in his paper).[9][10]

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.[11]

Cavendish's equipment was remarkably sensitive for its time.[9] The force involved in twisting the torsion balance was very small, 1.74×10−7 N,[12] about ​1⁄50,000,000 of the weight of the small balls.[13] To prevent air currents and temperature changes from interfering with the measurements, Cavendish placed the entire apparatus in a wooden box about 2 feet (0.61 m) thick, 10 feet (3.0 m) tall, and 10 feet (3.0 m) 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 0.16 inches (4.1 mm).[14] 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.[15] The accuracy of Cavendish's result was not exceeded until C. V. Boys's 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.[16]

Cavendish's result was also the first evidence for a planetary core made of metal. The result of 5.4 g·cm−3 is close to 80% of the density of liquid iron, and 80% higher than the density of the Earth's outer crust, suggesting the existence of a dense iron core.[17]

Whether Cavendish determined G

The formulation of Newtonian gravity in terms of a gravitational constant did not become standard until long after Cavendish's time. Indeed, one of the first references to G is in 1873, 75 years after Cavendish's work.[18]

Cavendish expressed his result in terms of the density of the Earth. For this reason, historians of science have argued that Cavendish did not measure the gravitational constant.[19][20][21][22] He referred to his experiment in correspondence as 'weighing the world'. Later authors reformulated his results in modern terms.[23][24][25]

After converting to SI units, Cavendish's value for the Earth's density, 5.448 g cm−3, gives

G = 6.74×10−11 m3 kg–1 s−2,

which differs by only 1% from the 2014 CODATA value of 6.67408×10−11 m3 kg−1 s−2.[26] Today, physicists often use units where the gravitational constant takes a different form. The Gaussian gravitational constant used in space dynamics is a defined constant and the Cavendish experiment can be considered as a measurement of this constant. In Cavendish's time, physicists used the same units for mass and weight, in effect taking g as a standard acceleration. Then, since Rearth was known, ρearth played the role of an inverse gravitational constant. The density of the Earth was hence a much sought-after quantity at the time, and there had been earlier attempts to measure it, such as the Schiehallion experiment in 1774.

For these reasons, physicists generally do credit Cavendish with the first measurement of the gravitational constant.[27][28][29][30][31]

Derivation of G and the Earth's mass

The following is not the method Cavendish used, but describes how modern physicists would calculate the results from his experiment.[30][32][33] From Hooke's law, the torque on the torsion wire is proportional to the deflection angle θ of the balance. The torque is κθ where κ is the torsion coefficient of the wire. However, a torque in opposite direction is also generated by the gravitational pull of the masses. It can be written as a product of the attractive forces between the balls and the distance to the suspension wire. Since there are two pairs of balls, each experiencing force F at a distance L/2 from the axis of the balance, the torque is LF. At equilibrium (when the balance has been stabilized at an angle θ), the total amount of torque must be zero, as these two sources of torque cancel out. Thus, we can equate their intensities given by the formulas above, which gives the following:

For F, Newton's law of universal gravitation is used to express the attractive force between the large and small balls:

Diagram of torsion balance

Substituting F into the first equation above gives

To find the torsion coefficient (κ) of the wire, Cavendish measured the natural resonant oscillation period T of the torsion balance:

Assuming the mass of the torsion beam itself is negligible, the moment of inertia of the balance is just due to the small balls:

,

and so:

Solving this for κ, substituting into (1), and rearranging for G, the result is:

Once G has been found, the attraction of an object at the Earth's surface to the Earth itself can be used to calculate the Earth's mass and density:

Definitions of terms
SymbolUnitDefinition
θradiansDeflection of torsion balance beam from its rest position
FNGravitational force between masses M and m
Gm3 kg−1 s−2Gravitational constant
mkgMass of small lead ball
MkgMass of large lead ball
rmDistance between centers of large and small balls when balance is deflected
LmLength of torsion balance beam between centers of small balls
κN m rad−1Torsion coefficient of suspending wire
Ikg m2Moment of inertia of torsion balance beam
TsPeriod of oscillation of torsion balance
gm s−2Acceleration of gravity at the surface of the Earth
MearthkgMass of the Earth
RearthmRadius of the Earth
ρearthkg m−3Density of the Earth

References
  1. Boys 1894 p. 355
  2. Poynting, John Henry (1911). "Gravitation" . In Chisholm, Hugh (ed.). Encyclopædia Britannica. 12 (11th ed.). Cambridge University Press. 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).
  3. Many sources incorrectly state that this was the first measurement of G (or the Earth's density); for instance: Feynman, Richard P. (1963). "7. The Theory of Gravitation". mainly mechanics, radiation and heat. The Feynman lectures on physics. Volume 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)(Encyclopædia Britannica 1910).
  4. Clotfelter 1987, p. 210
  5. 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.
  6. Cavendish 1798, p. 59 Cavendish gives full credit to Michell for devising the experiment
  7. 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
  8. Cavendish 1798, p.59
  9. Poynting 1894, p.45
  10. Chisholm, Hugh, ed. (1911). "Cavendish, Henry" . Encyclopædia Britannica. 5 (11th ed.). Cambridge University Press. pp. 580–581.
  11. Cavendish 1798, p.64
  12. Boys 1894 p.357
  13. Cavendish 1798 p. 60
  14. Cavendish 1798, p. 99, Result table, (scale graduations = ​1⁄20 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.
  15. Cavendish 1798, p.63
  16. Jungnickel & McCormmach 1996, p.341
  17. see e.g. Hrvoje Tkalčić, The Earth's Inner Core, Cambridge University Press (2017), p. 2.
  18. Cornu, A.; Baille, J. B. (1873). "Détermination nouvelle de la constante de l'attraction et de la densité moyenne de la Terre" [New Determination of the Constant of Attraction and the Average Density of Earth]. C. R. Acad. Sci. (in French). Paris. 76: 954–958.
  19. Clotfelter 1987
  20. Jungnickel & McCormmach 1996, p.337
  21. "Hodges 1999". Archived from the original on 2017-09-06. Retrieved 2006-03-07.
  22. Lally 1999
  23. Boys 1894, p.330 In this lecture before the Royal Society, Boys introduces G and argues for its acceptance
  24. Poynting 1894, p.4
  25. MacKenzie 1900, p.vi
  26. Lee, Jennifer Lauren (November 16, 2016). "Big G Redux: Solving the Mystery of a Perplexing Result". NIST.
  27. Halliday, David; Resnick, Robert (1993). Fundamentals of Physics. John Wiley & Sons. p. 418. ISBN 978-0-471-14731-2. Retrieved 2013-12-30. 'The apparatus used in 1798 by Henry Cavendish to measure the gravitational constant'
  28. Feynman, Richard P. (1963). Lectures on Physics, Vol.1. Addison-Wesley. pp. 6–7. ISBN 978-0-201-02116-5. 'Cavendish claimed he was weighing the Earth, but what he was measuring was the coefficient G...'
  29. Feynman, Richard P. (1967). The Character of Physical Law. MIT Press. pp. 28. ISBN 978-0-262-56003-0. 'Cavendish was able to measure the force, the two masses, and the distance, and thus determine the gravitational constant G.'
  30. "Cavendish Experiment, Harvard Lecture Demonstrations, Harvard Univ". Retrieved 2013-12-30.. '[the torsion balance was]...modified by Cavendish to measure G.'
  31. Shectman, Jonathan (2003). Groundbreaking Experiments, Inventions, and Discoveries of the 18th Century. Greenwood. pp. xlvii. ISBN 978-0-313-32015-6. Retrieved 2013-12-30. 'Cavendish calculates the gravitational constant, which in turn gives him the mass of the earth...'
  32. Poynting 1894, p.41
  33. Clotfelter 1987 p.212 explains Cavendish's original method of calculation

Sources
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