Cavendish Experiment

Henry Cavendish used a torsion balance, a horizontal rod hung from a thin wire with small lead balls at each end, to measure the tiny gravitational pull between the small balls and a pair of much larger stationary lead balls placed nearby. The twist in the wire told him how strong that pull was, and because he knew every mass and distance involved, he could apply Newton's law of universal gravitation directly to work out the gravitational constant and, from it, the density of the Earth. It was the first time gravity between ordinary sized objects had been measured in a laboratory rather than inferred from the motion of planets, and it confirmed that Newton's law holds at that much smaller scale too.