Charles-Augustin de Coulomb

Who Weighed A Force With A Twisted Wire · Engineer & Physicist · 1736–1806

The military engineer who built a balance so delicate that a twisted wire could weigh the force between two specks of charge — and found it falls off as the square of the distance, exactly as gravity does.

Who was Coulomb?

Charles-Augustin de Coulomb was born at Angoulême in 1736, trained at the school of military engineering at Mézières, and spent nine years in Martinique supervising the construction of Fort Bourbon, which ruined his health and gave him a long apprenticeship in what materials actually do. His first prize-winning memoirs are on the strength of structures and on the theory of friction, work that remained the standard account of dry friction for a century and a half. He then turned to the torsion balance. His crucial preliminary was not the electricity at all but the wire: he established, by timing the oscillations of a loaded wire, that the restoring couple it exerts is exactly proportional to the angle through which it has been twisted, so that an angle read off a scale IS a measurement of force. Only then did he trust it with a question. In the memoirs of 1785 to the Académie des Sciences he charged a small gilt pith ball fixed in the apparatus and another on the end of the suspended needle, let them repel, and read the twist: halve the separation and the force is four times as great. He obtained the same inverse-square dependence for magnetic poles. He also measured how charge divides between two identical spheres brought into contact, and how it leaks away along imperfect insulating supports and through damp air. He worked with two electric fluids, vitreous and resinous, and said plainly that he used them as a way of calculating rather than as a claim about what exists.

Major achievements

• Established the inverse-square law of force between electric charges, published in 1785
• Proved that a wire's restoring couple is proportional to the twist, making the torsion balance a measuring instrument
• Found the same inverse-square dependence for magnetic poles
• Showed that two identical touching spheres share their charge equally
• Measured the leakage of charge along supports and through moist air

Life in brief

• 1736 — Born at Angoulême: Trained at Mézières as a military engineer.
• 1764 — Martinique: Nine years building Fort Bourbon; his health never recovered.
• 1773 — Memoir on the strength of structures: Statics applied to real materials.
• 1781 — The theory of friction: A prize memoir that stood for a century and a half.
• 1785 — The torsion balance memoirs: The inverse-square law of electric force, measured.
• 1806 — Dies at Paris: The unit of charge is named after him in 1881.

Explore the life of Coulomb — five chapters

1. Nine Years at Fort Bourbon — An engineering education that cost his health
2. The Theory of Friction — A prize memoir that stood for a century and a half
3. Prove the Instrument First — The torsion balance, and why it can be trusted
4. Halve the Distance, Quadruple the Force — The memoirs of 1785
5. The Revolution, and a Unit — Retirement, return, and 1881

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