Georg Simon Ohm, a German physicist and mathematician, was born Mar. 16, 1789. Ohm was attracted to the mathematical physics of Joseph Fourier, who had recently produced a complete mathematical theory of heat, but Ohm was also interested in the brand-new discoveries by Ampere and Oersted that electricity and magnetism are related. In 1825 and 1826, Ohm did an elegant set of experiments in which he tried to determine how the thickness and length of a wire, and the metal from which it is made, affects its ability to conduct electricity. Using a compass needle placed near a wire as a measure of electrical current (ammeters had not yet been invented), Ohm discovered that, for a given wire, the current through the wire is proportional to the strength of the battery (we would say the voltage), but that for other wires, the current might be different for the same voltage. He formulated this as a law: the voltage divided by the current is equal to a quantity that we now call “resistance”. This law, universally recognized as Ohm’s Law, can be written more simply as V = IR, where I is the current.
Ohm’s law was announced in 1827 in a book titled Die galvanische Kette, mathematisch bearbeitet (The electrical circuit, mathematically determined). We do not have Die galvinische Kette in the History of Science Collection; it is one of those odd books that is always available on the market, but always over-priced; it is hard to justify spending $30,000 on Ohm’s book when Ampere’s book, equally important, carries a price tag half that size.
When Germany issued a series of postage stamps in 1994, commemorating famous (German) scientific discoveries, Ohm’s law was one of the honorees (first image). The title page of Ohm’s book (second image) is from a copy currently being offered for sale. The statue of Ohm is in a square in Munich (third image).
Dr. William B. Ashworth, Jr., Consultant for the History of Science, Linda Hall Library and Associate Professor, Department of History, University of Missouri-Kansas City. Comments or corrections are welcome; please direct to firstname.lastname@example.org.