The Man and the Machine
In 1798, on Clapham Common in south London, the Honourable Henry Cavendish conducted one of the most delicate physics experiments ever performed. Cavendish was an reclusive and eccentric figure, known for communicating with his female servants only through written notes and adding a private staircase to his house to avoid them. He lived for science, converting his Clapham mansion into a vast laboratory and observatory. Here, in a dedicated outbuilding in his garden, he set up an apparatus to measure the density of the Earth.
The device was a torsion balance, invented by his friend, the geologist John Michell, who died before he could use it. The apparatus consisted of a six-foot (1.8 m) wooden rod suspended horizontally by a thin wire. A 2-inch diameter (51 mm) lead sphere weighing 1.61 pounds (0.73 kg) was attached to each end. Two much larger lead balls, each weighing 348 pounds (158 kg), were suspended from a separate frame. These could be positioned near the smaller spheres. The entire assembly was housed in a wooden case inside a closed room to protect it from air currents and temperature changes. Cavendish made his observations from outside through telescopes aimed through holes in the shed walls, preventing his own body heat from disturbing the measurement.
A Weighty Result
The experiment's goal was to measure the faint gravitational attraction between the large and small lead balls. This tiny force, about 1/50,000,000th of the weight of the small balls, caused the wooden rod to rotate and twist the wire it hung from. By measuring the angle of this deflection, a mere 0.16 inches (4.1 mm) at its maximum—Cavendish could calculate the force between the spheres. From this, he could determine the Earth's density. He did not try to calculate the gravitational constant, G; that formulation came much later. He simply wanted to "weigh the world".
After 17 meticulous observations, Cavendish calculated the Earth's density to be 5.448 times that of water. This result is within 1.2% of the currently accepted value of 5.515 g/cm³. A small arithmetic error in his original paper, later corrected by Francis Baily in 1821, initially published the value as 5.48. The accuracy of his measurement was so great that it was not improved upon for nearly a century. The experiment was the first to measure the force of gravity between masses in a laboratory and provided the first accurate values for the Earth's mass and density. The original house and laboratory were demolished in 1905, and the site is now occupied by modern residential streets.