Heinrich Friedrich Emil Lenz

Heinrich Lenz was born in 1804 in Dorpat (now Tartu, Estonia), then a German-speaking university town within the Russian Empire. He entered the University of Dorpat at eighteen to study chemistry and physics, but his academic career took an unexpected turn when, at twenty-two, he was invited to serve as the physicist on Otto von Kotzebue's third circumnavigation of the globe aboard the Russian sloop *Predpriyatiye* (1823–1826). For three years Lenz measured seawater salinity, ocean temperatures at depth, and atmospheric pressure across the Pacific and Atlantic; his data — some of the first systematic oceanographic measurements ever taken — were still being cited a century later. He returned to St. Petersburg in 1828, joined the Academy of Sciences the following year, and in 1836 became professor of physics at St. Petersburg University, where he spent the rest of his life.

Lenz's 1834 paper "Über die Bestimmung der Richtung der durch elektrodynamische Vertheilung erregten galvanischen Ströme" ("On the Determination of the Direction of Galvanic Currents Excited by Electrodynamic Distribution") gave the rule that now carries his name: the direction of an induced current is always such that the magnetic field it produces opposes the change in flux that caused the induction. Faraday had already given the quantitative relationship — EMF proportional to rate of change of flux — but had treated the sign as an experimental convention. Lenz recognised it as a consequence of energy conservation: if the induced current reinforced the change, a small perturbation would feed back into itself and grow without bound, violating energy conservation. Instead, the induced current must subtract energy from the changing flux, turning the induction process into the electromagnetic analogue of inertia. The minus sign in Faraday's law — EMF = −dΦ/dt — is Lenz's contribution, and every textbook derivation of energy conservation in induced circuits routes through his argument.

Independently of James Prescott Joule, Lenz also established in 1842 that the heat dissipated in a resistor is proportional to the square of the current and to the resistance — the relation now known either as Joule's law or the Joule–Lenz law depending on tradition (Germans, Russians, and much of continental Europe call it Joule–Lenz; Anglophones call it Joule's law and usually omit Lenz). Lenz's measurements, made with self-built precision calorimeters, were actually more accurate than Joule's early results, but Joule published first and with greater fanfare. Lenz remained scientifically active into his sixties, holding the deanship of the physics faculty at St. Petersburg and serving as rector of the university from 1863. In January 1865, while on a European holiday in Rome, he suffered a fatal stroke while walking on the cliffs above the city; he was sixty-one. The rule of electromagnetic induction that bears his name is, in its simplest form, the principle every engineer uses to design eddy-current brakes, induction cookers, and regenerative motor systems: push against a magnetic change and the magnetic change pushes back.