Kirchhoff, Gustav Robert (1824–1887)

German physicist who with R W von Bunsen developed spectroscopic analysis in the 1850s and showed that all elements, heated to incandescence, have their individual spectra.

Kirchhoff was born and educated in Königsberg (now Kaliningrad). In 1850 he was appointed professor at Breslau, where he was joined by Bunsen the following year. In 1852 Bunsen moved to Heidelberg and Kirchhoff followed him 1854. He moved to Berlin 1875.

In 1845 and 1846 he derived the laws known as Kirchhoff's laws that determine the value of the electric current and potential at any point in a network. He went on to show that electrostatic potential is identical to tension, thus unifying static and current electricity.

Observing the dark lines in the Sun's spectrum, first discovered by German physicist Joseph Fraunhofer, Kirchhoff came to the conclusion that the Fraunhofer lines are due to the absorption of light by sodium and other elements present in the Sun's atmosphere.

In 1859 Kirchhoff announced a law stating that the ratio of the emission and absorption powers of all material bodies is the same at a given temperature and a given wavelength of radiation produced. From this, Kirchhoff went on in 1862 to derive the concept of a perfect black body – one that would absorb and emit radiation at all wavelengths.