Weighing the World: Cavendish’s Quiet Triumph in Measuring Gravity

In Chapter 7-6 of The Feynman Lectures on Physics, Richard Feynman poses a wonderfully straightforward question: if gravitation reaches across the void between planets and stars, why can’t we simply measure it here on Earth by placing a marble next to a heavy ball of lead and watching them draw together? The answer, as he explains, is that the gravitational force between such objects is vanishingly small. Detecting it demands extraordinary care: sealing the apparatus against air currents, avoiding any electrical charge, and measuring movements so tiny they flirt with invisibility.

The man who first succeeded in this endeavour was Henry Cavendish, whose famous experiment was designed to measure the strength of gravity directly between laboratory-sized masses. By doing so, he could determine the gravitational constant G, the last unknown in Newton’s universal law of gravitation, and from that deduce the mass of the Earth itself. Cavendish called it “weighing the Earth,” though in truth he was weighing the strength of the force that binds us to it.

The origins of the experiment go back further than Cavendish. The idea of using a torsion balance—a horizontal rod suspended by a fine wire, free to twist under tiny forces—came from the geologist and natural philosopher John Michell in the early 1780s. Michell died before he could build it, and his apparatus passed into Cavendish’s hands. It was Cavendish who perfected the design and carried out the meticulous measurements, publishing his results in 1798.

The setup was deceptively simple in appearance but an engineering marvel in execution. Inside a sealed wooden shed, Cavendish suspended a light rod, about 1.8 metres long, with a small lead sphere fixed to each end. Outside the shed, two much larger lead spheres could be swung into place beside the smaller ones. Their faint gravitational pull twisted the rod, twisting the suspension fibre with it. The movement was minute—fractions of a millimetre, corresponding to angles less than a single arcsecond—but Cavendish’s instruments could register it. He took his readings through telescopes aimed at vernier scales, ensuring he could work without disturbing the apparatus.

From the amount of twist in the fibre, Cavendish calculated the force between the masses. Knowing the masses and their separation, he could solve for G in Newton’s law. With that in hand, he calculated the density of the Earth to be about 5.45 times that of water—remarkably close to modern values—and so provided the first reliable figure for the Earth’s mass. His work also hinted at the Earth’s dense metallic interior, a conclusion that would later be borne out by seismology.

The implications of this experiment were profound. For the first time, Newton’s law of gravitation had been tested and confirmed in the laboratory, without recourse to astronomical observations. It showed that the same law governing the motion of the Moon and the fall of an apple could be measured between lumps of metal in a room. In doing so, it strengthened confidence in the idea that nature’s laws were universal and precise, encouraging physicists to search for equally simple principles behind other phenomena.

Cavendish’s method remained the standard way to measure G for over a century, and variations of the torsion balance are still used in modern physics—whether in precision tests of the equivalence principle, in searches for hypothetical “fifth forces,” or in experiments probing gravity at millimetre and sub-millimetre scales. The spirit of his work lives on in every laboratory where patience, precision, and a willingness to measure the almost immeasurable lead to a clearer understanding of the universe.