Gustav Kirchhoff, researched black body radiation emission and co-discovered caesium and rubidium

March 12, 1824, Königsberg (now Kaliningrad) - October 17, 1887, Berlin (Germany)

Gustav Robert Kirchhoff, from an early age, showed academic faculties, which led him to enroll at Albertus University (founded in 1544 by Albert, the first Duke of Prussia). There, he attended a three-year seminar in mathematical physics to introduce his students to research methods. During this time, he became interested in electrical induction and electric currents.

He graduated in 1847 and moved to Berlin in a tense situation, mainly due to the poor conditions in the German Confederation. There was unrest and Louis Philippe I of France was dethroned by an uprising in 1848, which led to revolutions in several German states and conflicts in Berlin.

He worked as an unpaid teacher from 1848 to 1950, rectifying what was previously believed about electric and electrostatic currents.

He is appointed extraordinary professor at the University of Wroclaw (now Wroclaw) and moves to another city. In the same year, he solves several problems concerning the deformation of elastic plates and meets the chemist Robert Bunsen, becoming friends.

In 1854, Bunsen was working in Heidelberg and offered Kirchhoff to join him as a professor of physics. From then on, they worked together very fruitfully.

In 1862, Kirchhoff proposed the name ‘blackbody radiation’ and postulated two sets of fundamental laws, in classical electrical circuit theory and in thermal emission. Although both are known as ‘Kirchhoff's Laws’, this name is probably more common in the field of electrical engineering:

1. Kirchhoff's first law or law of knots (or nodes): the sum of currents entering a knot is equal to the sum of those leaving (All incoming and outgoing currents in a knot add up to 0). For a metal, in which the charge carriers are electrons, the above statement is equivalent to saying that the electrons entering a node at a given instant are numerically equal to those leaving. Knots do not accumulate charge (electrons).
2. Kirchhoff's Second Law or the law of meshes: the sum of voltage drops in a section between two nodes is equal to the sum of voltage drops in any other section between these nodes.

His research on blackbody radiation was fundamental to the development of quantum theory. The astronomer and physicist Joseph von Fraunhofer had observed the bright lines in the spectrum produced by flames and noticed that they appeared at similar frequencies to the dark lines in the spectrum of the Sun. To make further progress, pure forms of these substances were required, as containing impurities produced a confused picture of the lines. Kirchhoff was able to make this important breakthrough, producing the pure forms of the substances studied, and in 1859 he was able to realise that each element had unique characteristics in the spectrum. He presented his radiation law stating what he discovered, saying that for a given atom or molecule, the emission and absorption frequencies are the same.

He proposed the three empirical laws describing the emission of light by incandescent objects:

• A hot solid object produces light in a continuous spectrum.
• A faint gas produces light with spectral lines at discrete wavelengths that depend on the chemical composition of the gas.
• A solid object at a high temperature surrounded by a faint gas at lower temperatures produces light in a continuous spectrum with gaps at discrete wavelengths whose positions depend on the chemical composition of the gas.

The justification of these laws was later given by the physicist Niels Bohr, contributing decisively to the birth of quantum mechanics.

In 1861, Kirchhoff and Bunsen studied the spectrum of the Sun, identifying the chemical elements in the solar atmosphere and discovering two new elements in the course of their research, cesium and rubidium.

Kirchhoff is known for being the first to explain the dark lines in the Sun's spectrum as the result of the absorption of particular wavelengths as light passes through the gases in the solar atmosphere, thereby revolutionising astronomy.

A medida que su salud empeoraba debido a una discapacidad que le obligaba a pasar gran parte de su vida en muletas o en silla de ruedas, le resultaba más difícil practicar la experimentación, por lo que cuando en 1875 le ofrecieron la cátedra de física matemática en Berlín, la aceptó para poder continuar haciendo contribuciones a la enseñanza y realizar investigación teórica.