Gustav Kirchhoff

Gustav Robert Kirchhoff (German: [ˈgʊstaːf ˈʁoːbɛʁt ˈkɪʁçhɔf]; 12 March 1824 – 17 October 1887) was a German physicist and mathematician who contributed to the fundamental understanding of electrical circuits, spectroscopy, and the emission of black-body radiation by heated objects.^[3][4] He coined the term black body in 1860.^[5]

Gustav Kirchhoff
Born	Gustav Robert Kirchhoff (1824-03-12)12 March 1824 Königsberg, East Prussia, Kingdom of Prussia
Died	17 October 1887(1887-10-17) (aged 63) Berlin, German Empire
Resting place	Alter St.-Matthäus-Kirchhof, Berlin
Alma mater	University of Königsberg (Dr. phil.)
Known for	Kirchhoff's law of thermal radiation Kirchhoff's circuit laws Kirchhoff's theorem Kirchhoff's diffraction formula Discovering caesium and rubidium
Spouses	Clara Richelot ​ ​ (m. 1857; died 1869)​ Luise Brömmel ​ (m. 1872)​
Children	5
Awards	Rumford Medal (1862) Cothenius Medal (1876) Matteucci Medal (1877) Davy Medal (1877) Janssen Medal (1887)
Scientific career
Fields	Thermodynamics, electromagnetism, graph theory, optics, spectroscopy
Institutions	University of Berlin (1847–1850, 1875–1887) University of Breslau (1850–1854) University of Heidelberg (1854–1875)
Thesis	Ueber den Durchgang eines elektrischen Stromes durch eine Ebene, insbesondere durch eine kreisförmige (1847)
Doctoral advisor	Franz Ernst Neumann
Notable students	See list Hermann Aron[1] Ivan Borgman[2] Siegfried Czapski[2] Loránd Eötvös[1] Heinrich Hertz[2] Heike Kamerlingh Onnes[2] Viktor Kirpichov[1] Johannes Knoblauch[1] Sofia Kovalevskaya[2] Gabriel Lippmann[1] Jacob Lüroth[1] Edward Leamington Nichols[2] Max Noether[1] Karol Olszewski[2] Max Planck[2] Ernst Schröder[1] Arthur Schuster[1] August Witkowski[2] Heinrich Martin Weber[1] Arthur Williams Wright[2]

Several different sets of concepts are named "Kirchhoff's laws" after him, which include Kirchhoff's circuit laws, Kirchhoff's law of thermal radiation, Kirchhoff's diffraction formula, and Kirchhoff's law of thermochemistry.

The Bunsen–Kirchhoff Award for spectroscopy is named after Kirchhoff and his colleague, Robert Bunsen.

Biography

Gustav Robert Kirchhoff was born on 12 March 1824 in Königsberg, Prussia, the son of Friedrich Kirchhoff, a lawyer, and Johanna Henriette Wittke.^[6] His family were Lutherans in the Evangelical Church of Prussia.

Kirchhoff studied at the University of Königsberg, where he attended the mathematico-physical seminar directed by C. G. J. Jacobi,^[7] Franz Ernst Neumann, and Friedrich Julius Richelot. In 1845, while a student, Kirchhoff formulated two circuit laws—which are now ubiquitous in electrical engineering. He completed this study as a seminar exercise; it later became his doctoral thesis, supervised by Neumann.

In 1847, Kirchhoff graduated from the University of Königsberg and became a Privatdozent (unsalaried lecturer) at the University of Berlin, where he stayed until 1850 when he was offered a professorship at the University of Breslau. In 1854, he was called to the University of Heidelberg, where he collaborated with Robert Bunsen in spectroscopic work. In 1875, Kirchhoff returned to Berlin, where he remained until his death in 1887.

In 1857, Kirchhoff married Clara Richelot, the daughter of his mathematics professor Richelot; the couple had five children. In 1872, after Clara's death in 1869, he married Luise Brömmel.^[8]

In 1864, Kirchhoff was elected a Member of the American Philosophical Society.^[9] In 1884, he became a Foreign Member of the Royal Netherlands Academy of Arts and Sciences.^[10]

Kirchhoff died on 17 October 1887 in Berlin at the age of 63. He is buried at Alter St.-Matthäus-Kirchhof (Old St. Matthew's Cemetery) in Schöneberg, Berlin (just a few meters from the graves of the Brothers Grimm).

Research

Kirchhoff (left) and Robert Bunsen, c. 1850

In 1857, Kirchhoff calculated that an electric signal in a resistanceless wire travels along the wire at the speed of light.^[11][12]

In 1859, Kirchhoff proposed a law of thermal radiation, and gave a proof in 1861.

Together, Kirchhoff and Bunsen improved on Joseph von Fraunhofer's 1814 spectroscope, which Kirchhoff used to pioneer the identification of the elements in the Sun, showing in 1859 that the Sun contains sodium. Kirchhoff and Bunsen discovered caesium and rubidium in 1861.^[13]

Kirchhoff contributed greatly to the field of spectroscopy by formalizing three laws that describe the spectral composition of light emitted by incandescent objects, building substantially on the discoveries of David Alter and Anders Jonas Ångström. In 1862, he was awarded the Rumford Medal "for his researches on the fixed lines of the solar spectrum, and on the inversion of the bright lines in the spectra of artificial light".^[a]

Kirchhoff also contributed to optics, carefully solving the wave equation to provide a solid foundation for Huygens' principle (and correct it in the process).^[15][16]

Kirchhoff's circuit laws

Main article: Kirchhoff's circuit laws

Kirchhoff's first law At any node in an electrical circuit where current can branch, the sum of the currents leaving the node is equal to the sum of the currents entering the node. The second law is that the algebraic sum of the potential drops along a closed circuit, taken in any direction of flow, is equal to zero.^[17]

Kirchhoff's three laws of spectroscopy

See also: History of spectroscopy

Visual depiction of Kirchhoff's laws of spectroscopy

1. A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum.
2. A low-density gas excited to emit light will do so at specific wavelengths, and this produces an emission spectrum.
3. If light composing a continuous spectrum passes through a cool, low-density gas, the result will be an absorption spectrum.

Kirchhoff did not know about the existence of energy levels in atoms. The existence of discrete spectral lines had been known since Fraunhofer discovered them in 1814. That the lines formed a discrete mathematical pattern was described by Johann Balmer in 1885. Joseph Larmor explained the splitting of the spectral lines in a magnetic field known as the Zeeman Effect by the oscillation of electrons.^[18][19] These discrete spectral lines were not explained as electron transitions until the Bohr model of the atom in 1913, which helped lead to quantum mechanics.

Kirchhoff's law of thermal radiation

It was Kirchhoff's law of thermal radiation in which he proposed an unknown universal law for radiation that led Max Planck to the discovery of the quantum of action leading to quantum mechanics.

Kirchhoff's law of thermochemistry

See also: Standard enthalpy of reaction § Variation with temperature or pressure

Kirchhoff showed in 1858 that, in thermochemistry, the variation of the heat of a chemical reaction is given by the difference in heat capacity between products and reactants:

${\displaystyle \left({\frac {\partial \Delta H}{\partial T}}\right)_{p}=\Delta C_{p}}$.

Integration of this equation permits the evaluation of the heat of reaction at one temperature from measurements at another temperature.^[20][21]

Kirchhoff's theorem in graph theory

Kirchhoff also worked in the mathematical field of graph theory, in which he proved Kirchhoff's matrix tree theorem.

Works

• Gesammelte Abhandlungen (in German). Leipzig: Johann Ambrosius Barth. 1882.
• Vorlesungen über Electricität und Magnetismus (in German). Leipzig: Benedictus Gotthelf Teubner. 1891.
• Vorlesungen über mathematische Physik. 4 vols., B. G. Teubner, Leipzig 1876–1894.
  • Vol. 1: Mechanik. 1. Auflage, B. G. Teubner, Leipzig 1876 (online).
  • Vol. 2: Mathematische Optik. B. G. Teubner, Leipzig 1891 (Herausgegeben von Kurt Hensel, online).
  • Vol. 3: Electricität und Magnetismus. B. G. Teubner, Leipzig 1891 (Herausgegeben von Max Planck, online).
  • Vol. 4: Theorie der Wärme. B. G. Teubner, Leipzig 1894, Herausgegeben von Max Planck^[22]

See also

• Circuit rank
• Computational aeroacoustics
• Flame emission spectroscopy
• Spectroscope
• Kirchhoff Institute of Physics
• List of German inventors and discoverers

Notes

1. Kirchhoff's banker, on hearing that Kirchhoff had identified the elements present in the Sun, remarked "of what use is gold in the Sun if it cannot be brought to Earth?" Kirchhoff deposited his prize money (gold sovereigns) with the banker, saying "here is gold from the Sun."^[14]