Gustav Robert Kirchhoff (12.03.1824-17.10.1887) was a German physicist and a member of the Berlin Academy of Sciences (1875). He was born in Königsberg. He graduated from the University of Königsberg (1846) and was professor at the universities of Breslau (from 1850), Heidelberg (from 1854) and Berlin (from 1875).
His research concerned electricity, mechanics, optics, mathematical physics, the theory of elasticity and hydrodynamics. In 1845- 47 he discovered the regularities in the flow of an electric current in branched electric circuits (Kirchhoff's rules), and in 1857 he constructed a general theory of the motion of current in conductors. Together with R. Bunsen he developed in 1859 the method of spectral analysis and discovered the new elements caesium (1860) and rubidium (1861). He established (1859) one of the fundamental laws of thermal radiation, according to which the ratio of the emissive power of a body to its absorptive power does not depend on the nature of the radiating body (Kirchhoff's law), and he proposed (1862) the concept of the black body and gave a model of it. He discovered the reversal of spectra (1860), explained the origin of the Fraunhofer lines and suggested that the Sun consists of an incandescent liquid mass surrounded by an atmosphere of vapour.
He developed (1882) a rigorous theory of diffraction. He improved Poisson's theory of magnetism. He also studied the elasticity of solids, the vibrations of plates and discs, the shape of a free jet of liquid and the motion of a body in a liquid medium.
A member of the St Petersburg Academy of Sciences (1862).