In physics, the purpose of the torsion bar experiment is to estimate the gravitational constant. The torsion bar experiment was originally proposed by John Michell, who constructed a torsion bar apparatus, but Michell died without completing the experiment. After his death (1783) the apparatus passed to Francis John Hyde Wollaston , who gave it to Henry Cavendish. Cavendish rebuilt the apparatus, although 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 wooden rod with attached metal spheres to each end, suspended from a wire. Two 350-pound lead spheres 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 and moon, and therefore the planets.

A description of Cavendish's experiment and a summary of several similar experiments are given by the 1911 Encyclopedia Britannica.

References

• "Gravitation". Encyclopedia Britannica, 1911.

• B. E. Clotfelter. "The Cavendish experiment as Cavendish knew it." American Journal of Physics, 55:210 (1987).

External Links

• Gravitation (an on-line copy of the article from the 1911 Encyclopedia Britannica)

• A Brief Biography of Henry Cavendish (biographical and historical notes)