Charles Augustin Coulomb

1736-1806

French Physicist and Engineer

Charles Augustin Coulomb was one of eighteenth-century Europe's greatest engineers. However, he is better known for inventing the torsion balance and discovering the inverse-square laws of attraction and repulsion for both electric and magnetic phenomena.

Coulomb was born in Angoulême, France on June 14, 1736. His family moved to Paris where he attended lectures at the Collège Mazarin and Collège de France. Following his father to Montpellier, he joined the Société des Sciences de Montpellier (1757). He entered the Ecole du Génie at Mézières (1761) to train as a military engineer and graduated a First Lieutenant in the Corps du Génie. Coulomb served at Brest (1761-1764) before being posted to Martinique (1764-1772). French postings at Bouchain, Cherbourg, and Rochefort followed before his election to the Académie des Sciences (1781) assured him permanent residence in Paris. Henceforth, he devoted himself to fundamental researches in physics.

Coulomb presented numerous memoirs before and participated in hundreds of civil engineering, instrumentation, and machinery committee reports to the Academy. With the coming of the Revolution Coulomb was obliged to resign his commission (1791). He later withdrew from Paris (1793) but returned as a member of the new Institut de France (1795). In 1802 Napoleon appointed him inspector-general of Public Instruction, a position he held until his death.

Coulomb's early work was largely in mechanics, producing treatises dealing with structural supports, beam flexure and rupture, friction, soil mechanics, and ergonomics. He introduced the concept of the "thrust line" and provided a general analysis of its application in controlling for oblique forces in the construction of buildings. In considering friction and cohesion he applied Guillaume Amontons' (1663-1705) law that friction is proportional to load, noting the law does not strictly hold and that coefficients of friction vary from material to material (1773).

Coulomb's further researches on friction appeared in his prize-winning essay "Théorie des machines simples" (1781). His investigations of both static and dynamic frictional forces under various conditions allowed him to confirm Amonton's earlier work that friction is approximately proportional to the normal pressure. He extended this work by considering effects due to variations in load, material, lubrication, velocity, and more. Coulomb's work remained the standard until superseded by early twentieth-century molecular studies of friction.

In another Academy prize-winning essay, Coulomb first presented his work on torsion (1777). He demonstrated how torsional forces in thin threads are proportional to the angle of twist and that torsion suspension could be used to measure extremely small forces, which he succeeded in doing.

Coulomb adapted his torsion balance to study electric and magnetic phenomena. His instrument consisted of a silken thread supporting a carefully balanced, wax-covered straw to which a charged (or magnetized) sphere could be attached. Isolated within a glass tube whose circumference was appropriately marked, another charged (or magnetized) sphere would be brought near the balance and the straw's rotation observed. Forces equivalent to 10-5 grams could be thus measured.

Coulomb presented the results of his researches in a series of memoirs read before the Academy (1785-1791). In the first, he demonstrated that electrical forces of repulsion are inversely proportional to the square of the distance separating the charges (1785). In the next memoir he extended these results by showing that the inverse-square laws of attraction and repulsion hold for both electric and magnetic forces (1787). Although he established the inverse-square laws, Coulomb never experimentally demonstrated electric or magnetic forces are proportional to the products of either charges or pole strengths. In the remaining memoirs Coulomb showed that charge leakage is proportional to charge, and that static charge is distributed on conductor surfaces (not their interiors); and he produced a fully-developed magnetic theory.

STEPHEN D. NORTON