Coulomb , Charles Augustin de

(1736–1806) French physicist

Coulomb, who was born in Angoulême, France, was educated in Paris and then joined the army, serving as an engineer. He spent nine years in Martinique designing and building fortifications, returning to France because of ill health. On his return he accepted several public offices but with the coming of the revolution he withdrew from Paris, spending his time quietly and safely at Blois and devoting himself to science. He returned to public life under Napoleon, serving as an inspector of public instruction from 1802.

He made an early reputation for himself by publishing work on problems of statics and friction. Some of the concepts that he introduced and analyzed are still used in engineering theory, for example, the notion of a thrust line. This describes how a building must be constructed if it is to control the oblique force arising from such items as roof members. Coulomb gave a general solution to the problem.

He is however most widely remembered for his statement of the inverse square laws of electrical and magnetic attraction and repulsion published in 1785. The secret of his work was the invention of a simple but successful torsion balance, which he used with great experimental skill. It was so sensitive that a force equivalent to about 1/100000 of a gram could be detected. The balance consisted of a silken thread carrying a carefully balanced straw covered with wax. The straw, to which a charged sphere could be fixed, was free to rotate in a large glass tube that was marked in degrees around its circumference. He could now bring another charged ball within various distances of the rotatable straw and measure the amount of twist produced. By varying the distances involved and the nature and amount of the charge, Coulomb was able to deduce a number of laws. He stated his “fundamental law of electricity” as “the repulsive forces between two small spheres charged with the same sort of electricity is in the inverse ratio of the squares of the distances between the centers of the two spheres.” That is, two like charged bodies will repel each other and the force of that repulsion will fall off with the square of the distance separating them: if a body moves twice as far away the repulsive force will be four times weaker, if the body moves thrice as far away the repulsive force will be nine times weaker, and so on for any distance between them. Coulomb went on to show that the same form of law applies to magnetic as well as electrical attraction and repulsion. What is surprising about Coulomb (and his contemporaries) was an inability to see any relationship between electricity and magnetism. Despite having demonstrated that the two phenomena obey basically the same laws he insisted that they consisted of two distinct fluids.

Coulomb was immortalized by having the unit of electric charge named in his honor: the quantity of electricity carried by a current of one ampere in one second is a coulomb.