#295 1797 · Henry Cavendish · Science / measurementremove-the-observer
Cavendish weighed the Earth by hiding from his own experiment in the next room
the problem
The force he needed to measure was fainter than his own body heat
background
By the late 18th century, scientists understood that gravity acted between all masses, not just between the Earth and objects on it, but no one had directly measured the gravitational pull between two ordinary objects in a laboratory — the force involved was almost immeasurably small at any scale a room could hold. Getting a reliable number for the Earth's density, and from it the strength of gravity itself, meant detecting an attraction between two lead spheres that was a tiny fraction of the force needed to lift a feather.
At that scale of sensitivity, the equipment itself became the least of the problem: air currents from someone walking past, or even the heat radiating from an experimenter's own body standing near the apparatus, could exert more force on a sensitive torsion balance than the gravity it was built to detect. Simply building a more sensitive instrument wouldn't help if the person reading it kept contaminating the measurement just by being in the room.
the move
Cavendish built a torsion balance — a six-foot rod hung from a thin wire with a small lead sphere at each end — and positioned much larger, 350-pound lead spheres near them to pull the rod slightly via gravity, twisting the wire by a minuscule, measurable angle. He sealed the entire apparatus inside a wooden case in a closed shed on his estate and read the deflection through a telescope aimed through a hole in the shed wall from an adjoining room, so his own presence never disturbed the air or temperature around the balance while it measured.
the payoff
Cavendish's result put the Earth's average density at 5.448 times that of water, and the gravitational constant derived from his data — 6.74×10⁻¹¹ m³kg⁻¹s⁻² in modern units — came within about 1% of the value accepted today.
what came after
The Cavendish experiment is treated as a landmark in the history of experimental physics, both for the first laboratory measurement of gravity between ordinary masses and for the observational discipline it demonstrated: isolating the measuring instrument from the observer's own physical presence, a principle that runs through precision measurement to this day.
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references
- [1]Cavendish experimentWikipedia, 2025en.wikipedia.org
- [2]June 1798: Cavendish weighs the worldAmerican Physical Society, 2008aps.org