A balance built by someone else
The apparatus Henry Cavendish used in 1797 and 1798 was not originally his. It began as a torsion balance conceived by the geologist John Michell, intended specifically to detect the gravitational attraction between ordinary laboratory-scale masses, an effect assumed at the time to be almost too faint to measure at all. Michell died in 1793 before he could carry out the experiment, and the crated apparatus passed first to Francis John Hyde Wollaston and then to Cavendish, who rebuilt it largely to Michell’s original design. Cavendish, a notoriously reclusive figure who avoided company and reportedly communicated with his own household staff only by written note, brought an unusual degree of experimental patience to finishing a project someone else had started and never lived to complete.
Lead spheres and a twisting wire
At the centre of the device sat a six-foot horizontal wooden rod hung from a thin torsion wire, with a small lead sphere roughly two inches across fixed at each end. Two much larger lead spheres, about twelve inches across, could be swung close to the smaller ones, and their gravitational pull drew the small spheres toward them just enough to twist the suspension wire through a small angle, around 0.16 degrees in Cavendish’s measurements, or as little as 0.03 degrees using a stiffer wire. He enclosed the whole assembly to shield it from air currents and watched the deflection from outside the room through telescopes fitted with vernier scales, wary that even his own body heat nearby might disturb a signal that small.
A force smaller than a feather’s weight
The scale of what he was measuring was extraordinary for the period: the gravitational force between the spheres worked out to roughly 1.74 times ten to the minus seven newtons, about one fifty-millionth of the small spheres’ own weight, and Cavendish could still resolve the resulting deflection to better than a quarter of a millimetre. From that measurement, once a small arithmetic error in his published figure was later corrected, his data implied Earth’s average density at about 5.448 times that of water, within roughly one percent of the modern accepted value of 5.514. Converted into modern units, the same measurement yields a gravitational constant within about one percent of today’s accepted figure, an unusually close match for an experiment run without any of the tools later scientists would take for granted.
Earth is denser than its surface suggests
The result carried a specific implication about what lies beneath Earth’s surface. A density close to eighty percent of that of liquid iron is far higher than ordinary surface rock, which meant the planet had to contain a dense, metal-rich interior beneath its lighter outer layers, an early and indirect but genuine clue about Earth’s internal structure long before geophysics had any direct way to probe it. The measurement also confirmed that Newton’s law of gravitation held at laboratory scale between ordinary objects, not only for planets and moons, extending a theory previously tested only on astronomical bodies down to two lead spheres in a shed on Cavendish’s own property.
An arithmetic slip, caught decades later
Cavendish’s own published figure was not quite right: his data pointed to a density of 5.448, but the number he actually printed was 5.480, a straightforward arithmetic slip that went unnoticed until the astronomer Francis Baily identified and corrected it in 1821, more than two decades after the original publication. Cavendish also never described his result in terms of a gravitational constant at all; expressing Newtonian gravity through such a constant did not become standard practice until nearly a century later, so the now-familiar comparison between his measurement and the modern value of G is a later reinterpretation of results he originally reported purely as a statement about Earth’s density.
A constant nobody had named yet
Michell’s torsion-balance design, as completed and refined by Cavendish, went on to become the standard method for measuring gravitational attraction in the laboratory, and Cavendish’s own level of precision was not exceeded until C. V. Boys’ experiment in 1895, nearly a century later. Variations on the same basic approach, a thin wire, a suspended mass, a measured twist, remain central to most contemporary measurements of the gravitational constant. The story is worth an hour as much for that inherited apparatus, built by a man who never got to use it, as for the number it eventually produced.