The quantative study of electricty began with Charles A. Coulumb, who was the founder of electrostatics and the school of experimental physics in France. In 1785, with the aid of a small torsional balance electroscope, he estalibshed Coulumb's law by showing that the force of attraction or repulsion between two charges spheres varied inversely with the square of the distance between their centers. He served as a military engineer for France in the West Indies, but retired to Blois, France, at the time of the French Revolution to continue research in magnetism, friction, and electricity. In 1777 he invented the torsion balance for measuring the force of magnetic and electrical attraction. With this invention, Coulomb was able to formulate the principle, now known as Coulomb's law, governing the interaction between electric charges. In 1779 Coulomb published the treatise Théorie des machines simples (Theory of Simple Machines), an analysis of friction in machinery. After the war Coulomb came out of retirement and assisted the new government in devising a metric system of weights and measures. The unit of quantity used to measure electrical charges, the coulomb, was named for him.