Ampère

André-Marie Ampère was born in 1775 in Poleymieux-au-Mont-d'Or, near Lyon, into a prosperous family whose fortunes were destroyed by the French Revolution. His father, Jean-Jacques Ampère, was a justice of the peace and an admirer of Rousseau who believed in the principles of the Revolution—until Lyon rose against the National Convention in 1793. When Lyon fell to Republican forces, Jean-Jacques was arrested and guillotined in November 1793. André-Marie, eighteen years old, was devastated. According to his own account, he spent a year in a state of near-catatonic grief before slowly returning to mathematics, the discipline that had absorbed him since childhood.

Ampère had taught himself Latin to read Euler and Bernoulli in the original. By his twenties he was corresponding with the leading mathematicians of France, and in 1804 he was appointed to the École Polytechnique in Paris. His early work was in mathematics and chemistry, but in 1820 the Danish physicist Hans Christian Ørsted published the discovery that electric current deflects a compass needle—the first evidence that electricity and magnetism were related. Ampère heard about this result on September 11, 1820, and within a week had presented a mathematical framework explaining it. Within two months he had formulated the fundamental laws of electrodynamics.

Ampère's key insight was that electrical current flowing through a wire behaved like a magnet—and that two parallel wires carrying current attracted or repelled each other depending on the direction of current flow. He developed precise mathematical laws governing these forces. He coined the word 'electrodynamics,' distinguished between voltage (which he called 'tension') and current, and proposed what became Ampère's Law—a cornerstone of Maxwell's equations, published thirty years later. James Clerk Maxwell called Ampère 'the Newton of electricity.'

The ampere was defined as a unit of electrical current at the 1881 International Electrical Congress in Paris—the first international congress to standardize electrical units. It was defined as the amount of current that would deposit a specific amount of silver from a silver nitrate solution per second (a galvanic definition), and later redefined in terms of the force between parallel conductors. Since 2019, the SI system has defined the ampere in terms of the elementary charge—the charge of a single electron. Ampère himself never knew that current was the flow of electrons; electrons were not discovered until 1897. He described a phenomenon he could not fully explain at the particle level, and got it right anyway.