Cavendish experiment
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In physics, the Cavendish experiment was the first experiment to accurately measure the gravitational constant by measuring the force of gravity between two masses in the laboratory. The experiment was originally 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 Wollaston, 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. Two 350 pound (159 kg) 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, the moon, and the other planets.
Modern measurements of the gravitational constant still use variations of this method.
A description of Cavendish's experiment and a summary of several similar experiments are given by the 1911 Encyclopædia Britannica.
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)
- "Gravitation". Encyclopædia 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 Encyclopædia Britannica)
- A Brief Biography of Henry Cavendish (biographical and historical notes)
- Bending Spacetime in the Basement (do-it-yourself Cavendish apparatus)
- Cavendish Experiment description (Univ. of Saskatchewan student lab exercise with diagrams of apparatus)
- The Michell-Cavendish Experiment (paper by Iowa State Univ. professor Laurent Hodges describing Michell's contributions)
- Measuring Big G, J.H. Gundlach, Measurement Sci.& Tech. 10 454 (1999) (Recent experiment at the University of Washington using a variation of the Cavendish method)es:Experimento de la balanza de torsión