Coulomb’s torsion balance

I once demonstrated diamagnetism for the children’s TV series Gastronuts, bringing a magnet close to one of two grapes at the ends of a straw suspended by a slender thread; like magic, the grape moved gently away. After broadcast the channel received a furious letter from a medical doctor outraged by the ‘pseudoscientific magnetic nonsense’. Drafting a reply, I started thinking about the exquisite delicacy of the torsion balance, invented by one of those late 18th century pro-am scientists who did so much to change our understanding of the world.

Charles Augustin Coulomb was a military engineer, born into a well-to-do family near Angoulême, France. In 1764 he was sent to the island of Martinique, one of the last French colonial possessions in the Caribbean, whose fortress needed strengthening. For the next eight years Coulomb would direct the works on Fort Bourbon, suffering from periodic episodes of malaria. The works were an opportunity to experiment, and Coulomb explored issues related to the strength of building materials and the friction of materials on slopes.