Gustav Kirchhoff

Gustav Robert Kirchhoff (1824-1887) was a German physicist whose work in electrical circuits, spectroscopy, and thermal radiation placed him at the heart of nineteenth-century physical science. Born in Konigsberg in East Prussia, the son of a law councillor, he studied at the local university under Franz Neumann and Friedrich Richelot and completed his doctorate in 1847 on the conduction of electricity in plates.

While still a student he formulated the two laws of electrical circuits that bear his name, the current law and the voltage law, published in 1845 as a generalization of Ohm's results to networks of arbitrary topology. The laws remain the foundational tools of circuit analysis taught to every engineer.

He held professorships at Breslau and Heidelberg and, from 1875, the chair of mathematical physics in Berlin. At Heidelberg his collaboration with the chemist Robert Bunsen produced the principles of spectrum analysis. By matching the dark Fraunhofer lines in sunlight to bright lines emitted by heated elements in the laboratory, the pair showed in 1859 that the Sun and stars contain the same chemical elements as the Earth, and they discovered cesium and rubidium using the new technique.

His 1859 law of thermal radiation, stating that the ratio of emissive to absorptive power of a body depends only on its temperature and wavelength, defined the concept of the black body and set the problem whose solution by Planck in 1900 opened the quantum era.

The given name Gustav is a Germanic form of the Old Swedish name Gautstafr, combining Gautr, a Goth, with stafr, staff. He carried it through a career that joined elegant theory to careful measurement.