Gustav Robert Kirchhoff (March 12, 1824 – October 17, 1887), a German physicist who contributed to the fundamental understanding of electrical circuits, spectroscopy, and the emission of black-body radiation by heated objects. He coined the term "black body" radiation in 1862, and two sets of independent concepts in both circuit theory and thermal emission are named "Kirchhoff's laws" after him.

Gustav Kirchhoff was born in Königsberg, Prussia (now Kaliningrad, Russia), the son of Friedrich Kirchhoff, a lawyer, and Johanna Henriette Wittke. He graduated from the Albertus University of Königsberg (now Kaliningrad) in 1847 and married Clara Richelot, the daughter of his mathematics professor Friedrich Richelot. In the same year, they moved to Berlin, where he stayed until he received a professorship at Breslau (now Wroclaw).

Kirchhoff formulated his current law voltage law for circuit analysis, which are now ubiquitous in electrical engineering, in 1845, while still a student. He proposed his law of thermal radiation in 1859, and gave a proof in 1861. At Breslau, he collaborated in spectroscopic work with Bunsen, he was a co-discoverer of caesium and rubidium in 1861 while studying the chemical composition of the Sun via its spectral signature.

Later, he postulated three empirical laws describing the spectral composition of light emitted by incandescent objects.

1. A hot solid object produces light with a continuous spectrum.
2. A hot tenuous gas produces light with spectral lines at discrete wavelengths (i.e. specific colors) which depend on the energy levels of the atoms in the gas. (See also: emission spectrum)
3. A hot solid object surrounded by a cool tenuous gas (i.e. cooler than the hot object) produces light with an almost continuous spectrum which has gaps at discrete wavelengths depending on the energy levels of the atoms in the gas.

The existence of these discrete lines was later explained by the Bohr model, which helped lead to the development of quantum mechanics.

References

• Short biography (http://chem.ch.huji.ac.il/~eugeniik/history/kirchhoff.htm)
• Biography (http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Kirchhoff.html) at the MacTutor History of Mathematics Archive