GUSTAV ROBERT KIRCHHOFF (1847-74)

Physicist who with the German chemist Robert Bunsen firmly established the theory of spectrum analysis (a technique for chemical analysis by analyzing the light emitted by a heated material) which Kirchhoff applied to determine the composition of the Sun.

In 1845 he first announced Kirchhoff´s laws which allow calculations of the currents, voltages and resistances of electrical networks. Extending the theory of the German physicist Georg Simon Ohm he generalized the equations describing current flow to the case of electrical conductors in three dimensions. In further studies he demonstrated that current through a conductor at the speed of light.

In 1847 he became Privatdozent (unsalaried lecturer) at the University of Berlin and three years later accepted the post of extraordinary professor of physics at the University of Breslau, Prussia, now Wroclaw, Poland.

In 1854 he was appointed professor of physics at the University of Heidelberg where he joined forces with Bunsen and founded spectrum analysis. They demonstrated that every element gives off a characteristic coloured light when heated to incandescence. The light when separated by a prism has a pattern of bright lines (individual wavelengths of light) specific for each element. Applying this new research tool they discovered two new elements cesium (1860) and rubidium (1861).

Kirchhoff went further to apply spectrum analysis to study the composition of the Sun. He found that when light passes through a gas, the gas absorbs those wavelengths that it explain the numerous dark lines (Fraunhofer lines) in the Sun´s spectrum. That discovery marked the beginning of a new era in astronomy.

In 1875 he was appointed to the chair of mathematical physics at the University of Berlin. Most notable of his published works are Vorlesungen über mathematische Physik (4 vol., 1876-94, Lectures of Mathematical Physics) and his collected works Gesammelte Abhandlungen (1882, supplement; 1891, Collected Essays).