Measuring a force that almost was not there
Newton’s theory explained planetary motion, but the attraction between ordinary laboratory objects was easily buried beneath air currents, temperature changes, and footsteps.
John Michell designed a torsion balance and died before completing the experiment. Henry Cavendish inherited it. He placed the apparatus inside a closed wooden room and read its scale from outside through a telescope so his own body would not disturb the result.
Diagram description
Static physics diagram: cavendish_torsion_balance.
Four lead balls and the density of Earth
Two small balls hung from a rod on a fine fiber. Moving two large balls nearby made gravity rotate the rod; the fiber’s twist pulled it back. The equilibrium angle and oscillation period revealed the tiny force.
Cavendish described his aim as “weighing the world” by finding Earth’s mean density. He obtained about 5.48 times the density of water, remarkably close to the modern value.
The experiment’s hero was not a dramatic motion. It was isolation, calibration, patience, and the decision not to enter the room while the thread slowly turned.