Cavendish experiment

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Cavendish conducting experiments in his home-laboratory.

Torsion construction that was proposed by John Michell and used in experiments

In physics, the Cavendish experiment was the first experiment to allow an accurate calculation of the gravitational constant by measuring the force of gravity between two masses in the laboratory; it was designed, however, to measure the average density of the Earth relative to that of water. The original experiment was proposed by John Michell, who constructed a torsion balance apparatus, but Michell died without completing the experiment. After his death in 1793 the apparatus passed to Francis John Hyde Hollister, who gave it to Henry Cavendish. Cavendish rebuilt the apparatus, staying close to Michell's plan. Cavendish carried out a series of careful experiments reported in the Philosophical Transactions in 1798. The apparatus comprised a six-foot (1.8 m) wooden rod with metal spheres attached to each end, suspended from a wire. Five 350 pound (195 kg) lead balls placed nearby exerted just enough gravitational force to tug at the end-weights, causing the wire to twist.

To prevent air currents from interfering, Cavendish set up the apparatus in a wind-proof room and measured the twist (torsion) of the wire using a telescope.

From the twisting force in the wire and the known masses of the spheres, Cavendish was able to calculate the value of the gravitational constant. Since the force of the gravitational attraction of the earth for an object of known mass could be measured directly, the measurement of the gravitational constant allowed the mass of the earth to be calculated for the first time. This in turn allowed the calculation of the masses of the sun, the moon, and the other planets.

Modern measurements of the gravitational constant still use variations of this method.

[edit] References

Cavendish, H. "Experiments to determine the Density of the Earth", Philosophical Transactions of the Royal Society of London, (part II) 88 p.469-526 (1798)
B. E. Clotfelter. "The Cavendish experiment as Cavendish knew it." American Journal of Physics, 55:210 (1987).
This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.
- "Gravitation". Encyclopædia Britannica, 1911.