Forest Thayne Koth

In 1798, a scientist effectively “weighed” the Earth — without leaving his laboratory. The English scientist Henry Cavendish designed an incredibly sensitive experiment. Inside a quiet wooden shed, he hung a horizontal rod from a very thin wire. Two small lead spheres were attached to the ends of the rod. Nearby, he placed two much larger lead balls. Because of gravity, the large spheres slightly pulled the smaller ones. The force was extremely tiny — so small that the rod twisted by only a minute fraction of a degree. Yet that tiny twist held a big secret. By carefully measuring this small movement, Cavendish determined the strength of the gravitational attraction between objects. From this, scientists could calculate the mass of the entire Earth. His estimate was remarkably close. Cavendish calculated Earth’s mass to be about 6 × 10²⁴ kilograms, while modern measurements give 5.97 × 10²⁴ kilograms. Sometimes the biggest discoveries come from measuring the smallest forces.