Background

Gustav Robert Kirchhoff was born on March 12, 1824, in Konigsberg, Kingdom of Prussia (now Kaliningrad, Russia). He was the son of Friedrich Kirchhoff and Johanna Henriette Wittke. The father, a law councilor, belonged to the strongly disciplined body of state functionaries which also included university professors. He regarded it as a matter of course that his sons keep up, according to their diverse talents, the family’s allegiance to the service of the Prussian state.

Education

Kirchhoff entered the University of Konigsberg in 1842. Franz Neumann and Jacobi had jointly set up a mathematics-physics seminar at Konigsberg in 1833, and they used it to introduce their students to methods of research. Kirchhoff attended the Neumann-Jacobi seminar from 1843 to 1846. Now 1843 was the year in which Jacobi became unwell, so it was Neumann who influenced Kirchhoff in a very positive way. Neumann's interests were at this time firmly in mathematical physics and, at the time Kirchhoff began to study at Konigsberg, Neumann had become interested in electrical induction. In fact Neumann published the first of his two major papers on electrical induction in 1845 while Kirchhoff was studying with him. Later Kirchhoff was taught mathematics at the University of Konigsberg by Friedrich Jules Richelot.

Career

Kirchhoff taught at Berlin in an unpaid post from 1848 to 1850, and it was while he was working in Berlin that he corrected the accepted understanding of electric currents and electrostatics which we referred to above. He left Berlin for Breslau in 1850 when he was appointed as extraordinary professor there. In the year that he arrived in Breslau, Kirchhoff solved a problem concerning the deformation of elastic plates. An early form of the theory had been developed by Germain and Poisson but it was Navier who gave the correct differential equation a few years later. Problems remained, however, which Kirchhoff solved using variational calculus.

While Kirchhoff was in Breslau he met Bunsen who spent the academic year 1851-52 there; the two becoming firm and lasting friends. In 1854 Bunsen, who was working at Heidelberg, encouraged and supported Kirchhoff to move there. Kirchhoff accepted the offer of an appointment as professor of physics, and he began a fruitful collaboration with Bunsen. He shared in the academic and social excitement generated in Heidelberg by the circle gathered around Helmholtz.

In 1854, on Bunsen’s proposal, Kirchhoff was called to Heidelberg. He found there a congenial environment for his talents as teacher and investigator; and it was there that, partly in collaboration with Bunsen, he made his greatest contributions to science. This was the heyday of the university of Heidelberg, where the academic circle gathered around Helmholtz and, dominated by him, led a showy social life.

On two occasions he turned down calls to other universities; only when his failing health hindered his experimental work did he accept a chair of theoretical physics offered him in Berlin (1875). He took up this new task with great devotion, until illness forced him to give up his teaching activity in 1886.

Achievements

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  Gustav Robert Kirchhoff’s major contribution to physics was his experimental discovery and theoretical analysis in 1859 of a fundamental law of electromagnetic radiation: for all material bodies, the ratio of absorptive and emissive power for such radiation is a universal function of wavelength and temperature. Kirchhoff made this discovery in the course of investigating the optical spectra of chemical elements, by which, in collaboration with Bunsen, he laid the foundation of the method of spectral analysis. Outstanding among his other contributions were his early work on electrical currents and on the propagation of electricity in conductors.
  
  A master in the mathematical analysis of the phenomena, he insisted on the clear-cut logical formulation of physical concepts and relations, directly based on observation and leading to coherent systems free of hypothetical elements.
  
  His teaching had a considerable influence on the development in Germany of a flourishing school of theoretical physics during the first three decades of the twentieth century. The excellence of Kirchhoff as a teacher can be inferred from the printed text of his lectures. They set a standard for the teaching of classical theoretical physics in German universities, at a time when they were taking a leading position in the development of science.
  
  He received numerous honors and awards, including the Rumford Medal, the Matteucci Medal, the Janssen Medal, and the Davy Medal.

Works

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  • The laws of radiation and absorption 1901
  • On the Theory of Light Rays 1882
  • Gustav Robert Kirchhoff: Festrede zur Feier des 301 1887
  • Vorlesungen über mathematische Physik 1876
  • Abhandlungen Uber Mechanische Warmetheorie 1864
  • Gesammelte Abhandlungen 1891
  • Researches on the solar spectrum, and the spectra of the chemical elements 1863
  • Chemische Analyse durch Spectralbeobachtungen 1895
  • Chemische Analyse Durch Spectralbeobachtungen 1860
  • Anleitung Zur Deutschen Landes Und Volksforschung 1889
  • Abhandlungen Uber Mechanische Warmetheorie 1898
  • Graphische Behandlung der Schiebersteuerungen nach Zeuners Diagramm 1889
  • Mechanik 1897

Religion

Kirchhoff’s family were Lutherans in the Evangelical Church of Prussia.

Politics

Like other prominent figures of the German intelligentsia, Kirchhoff does not seem to have had difficulty in reconciling submission to authority in political matters with liberal opinions in other respects.

Views

Kirchhoff’s first scientific work dates from the time when he was studying under Neumann. One of the results he then arrived at has become, on account of its practical importance, a classical part of the theory of stationary electric currents: it is the formulation of the laws governing the distribution of tension and current intensity in networks of linear conductors (1845-1846). The derivation of these laws was essentially a simple application of Ohm’s law, but generalizing it fully, as the twenty-one-year-old student did, demanded uncommon mathematical skill.

Kirchhoff’s turn of mind was such that he was not long in discovering such a logical flaw and finding the way to mend it. In 1849 he was induced to look into the matter when confronted with some experiments by Kohlrausch on a closed circuit including a condenser, which involved both a static distribution and a flow of electricity. Kirchhoff pointed out that a consistent formulation of Ohm’s theory required the identification of the tension with the electrostatic potential. Thus a correct mathematical unification of electrostatics and the theory of voltaic currents was achieved after more than twenty years of neglect.

The theory of variable currents raised more difficult problems. The field was still open when Kirchhoff entered it in 1857 with his own general theory of the motion of electricity in conductors. His first paper, in which he treated linear conductors from the same premises as Weber, turned out to coincide in all essentials with an investigation carried out by Weber shortly before but delayed in publication. Both physicists noticed a remarkable implication of their theory: in a perfectly conducting circuit, oscillating currents could be propagated with a constant velocity, independent of the nature of the conductors, and numerically equal to the velocity of light. Both Kirchhoff and Weber, however, pointing to the extreme character of the condition of infinite conductivity, dismissed this result as a mere accidental coincidence.

In a second paper Kirchhoff presented a generalization of the theory to conductors of arbitrary shape. Although his equations purporting to give the local distribution of current and electromotive force were fundamentally wrong, they did yield for the total current the approximate equation already derived by William Thomson, and known as the “telegraphists’ equation” on account of its application to the propagation of current in the transatlantic cable then being laid.

Kirchhoff, aiming at a neat mathematical theory complete in itself, was operating with limited sets of concepts and relations directly suggested by experience. That he thus narrowly missed a great discovery illustrates the weakness inherent in his phenomenological method: emphasizing logical consistency entails the risk of closing the logical construction too soon and of overlooking possible connections between qualitatively different phenomena. In the case of voltaic currents, the closure of the theory demanded an extension of the scope of the potential concept, and the method led - by good luck - to a unification of two hitherto separated domains. But in electrodynamics the opposite happened. The ideal program of a physics in which the various forces of nature would be ascribed to specific, sharply separated types of action at a distance blinded its adherents to the strong hint of a possible similarity between the dynamics underlying optical and electromagnetic phenomena. Lorenz’ success, by contrast, resulted from his firm belief in the essential unity of all physical phenomena.

The events leading to the foundation and elaboration of the method of spectral analysis have been described by Bunsen. Bunsen was exploring the possibility of analyzing salts on the basis of the distinctive colors they gave to flames containing them; he had tried with some success to use colored pieces of glass or solutions to distinguish similarly colored flames. Kirchhoff pointed out that a much finer and surer distinction could be obtained from the characteristic spectra of such colored flames; unknown to him, the approach had been tried before, if only in a dilettantish way.

By rigorous experimentation, however, Bunsen and Kirchhoff soon put the method on a firm basis. The burner invented by Bunsen gave a flame of very high temperature and low luminosity, which emitted line spectra of great sharpness. The salts they investigated were prepared in a state of highest purity, and a spectroscope was specially designed to allow the positions of the lines to be accurately determined. By testing an extensive variety of chemical compounds, the ascription to each metal of its characteristic line spectrum was uniquely established (1860). The power and importance of spectral analysis became immediately apparent: its very first systematic application to alkali compounds led Bunsen to the discovery of two new alkaline elements, cesium and rubidium (1860).

In the course of his preparatory work in the autumn of 1859, Kirchhoff made an unexpected observation. It had long been known that the dark D lines, noticed in the solar spectrum by Fraunhofer (1814), coincided with the yellow lines emitted by flames containing sodium. Kirchhoff’s unexpected discovery was that if the intensity of the solar spectrum increased above a certain limit, the dark D lines were made much darker by the interposition of the sodium flame. He instantly felt that he had got hold of “something fundamental,” even though he was at a loss to suggest an explanation.

On the day following the surprising observation, Kirchhoff found the correct interpretation, which was soon confirmed by new experiments: a substance capable of emitting a certain spectral line has a strong absorptive power for the same line. In particular, the interposition of a sodium flame of low temperature is sufficient to produce artificially the dark D lines in the spectrum of an intense light source which did not show them originally. The dark D lines in the solar spectrum could accordingly be ascribed to absorption by a solar atmosphere containing sodium. Immense prospects thus opened up of ascertaining the chemical composition of the sun and other stars from the study of their optical spectra.

A few more weeks sufficed for Kirchhoff to elaborate a quantitative theory of the relationship between emissive and absorptive power. He attacked the problem directly by a wonderfully simple and penetrating argument. He considered the balance of radiative exchanges between bodies with appropriately chosen properties of absorption and emission. From the sole condition of radiative equilibrium at a given temperature, he was able to conclude that the ratio of absorptive and emissive powers, for each wavelength, must be independent of the nature of the bodies, and hence that it was a universal function of wavelength and temperature. In a later elaboration of the argument (1862), he introduced the conception of a “black body,” which absorbs completely every radiation incident on it. Since by definition the absorptive power of such a body has its maximum value, unity, for all wavelengths, its emissive power directly represents the universal function whose existence is asserted by Kirchhoff’s law. Hence, this function expresses the spectral distribution of the energy of radiation in equilibrium with a black body of given temperature; moreover, the empirical determination of this universal distribution is reduced to the practical problem of devising a material system with properties approximating those of a black body, and of measuring its emissive power.

Kirchhoff’s derivation of the fundamental law of radiative equilibrium is the triumph of his phenomenological method. He was fully aware of this methodological aspect and attached great importance to it. About ten years before the events just related, Stokes had commented to William Thomson on the coincidence of the Fraunhofer D lines and the bright lines of the sodium flame. Stokes suggested resonance as a mechanical explanation of this phenomenon: the sodium atom would have a proper frequency of vibration corresponding to that of the yellow light it emits and would accordingly absorb most intensively light of the same frequency. Now, Stokes’s suggestion, which appears to us a striking anticipation of the atomic basis of Kirchhoff’s law, did not appeal to Kirchhoff. When called upon to express an opinion on it (1862), Kirchhoff firmly asserted that the truth of the law had been established only by his own theoretical considerations and the supporting experiments; he thus implicitly denied Stokes’s argument any demonstrative value.

Yet, Kirchhoff was not averse to atomistic ideas. Whenever he judged the atomic substratum of phenomena to be sufficiently accessible to analysis, as in the kinetic theory of gases, he readily adopted the proposed atomistic picture. Fully sharing the common ideal of a purely mechanical description of the universe, he realized that such a description could be achieved only on the atomic scale; but he thought - with some reason - that the time was not ripe for it. For him, arguments depending on detailed and unwarranted assumptions about the structure and properties of atoms were without cogency in spite of their suggestiveness. Kirchhoff’s fidelity to the phenomenological point of view was thus dictated solely by methodological reasons; if this viewpoint sometimes proved too narrow, it nevertheless inspired not only his discoveries but his no less original attempt at a systematic exposition of the whole of physics. The historical importance of this attempt should not be underestimated.

Quotations: "Look here, I have succeeded at last in fetching some gold from the sun."

"Mechanics is a twin sister of geometry; both sciences are applications of pure mathematics; the propositions of both, as to their certainty, stand on the same level."

"If we were acquainted with all the forces of nature and knew what is the state of matter at a certain moment of time, we should be able to deduce by means of mechanics its state at every subsequent moment."

"We shall never attain the goal of the natural sciences, but even the fact that it is recognized as such offers a certain satisfaction, and in approximating to it lies the highest pleasure."

"That there are such limits to our knowledge of nature, must be borne with patience by every sound mind whether he be a scientist or a workman."

Membership

Kirchhoff was elected an Honorary Fellow of the Royal Society of Edinburgh, Foreign member of the Royal Society, and member of the Royal Netherlands Academy of Arts and Sciences, the Hungarian Academy of Sciences, the American Academy of Arts and Sciences, the Russian Academy of Sciences, the Royal Prussian Academy of Sciences, the Gottingen Academy of Sciences, and the Academy of Sciences of Turin.

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  Honorary Fellow

  Royal Society of Edinburgh , United Kingdom

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  Foreign member

  Royal Society , United Kingdom

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  Member

  Royal Netherlands Academy of Arts and Sciences , Netherlands

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  Member

  Hungarian Academy of Sciences , Hungary

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  Member

  American Academy of Arts and Sciences , United States

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  Member

  Russian Academy of Sciences , Russia

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  Member

  Royal Prussian Academy of Sciences , Germany

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  Member

  Gottingen Academy of Sciences , Germany

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  Member

  Academy of Sciences of Turin , Italy

Personality

Boltzmann described Kirchhoff, at the height of his powers, as being not easily drawn out but of a cheerful and obliging disposition. His poor state of health did not alter his cheerfulness, and he bore with patience the long illness of his last years.

Physical Characteristics: Kirchhoff was involved in an accident which compelled him to use crutches or a wheelchair.

Quotes from others about the person

• L. Rosenfeld: "In a period of expanding scientific horizons, the need soon arises for ordering and logical analysis of new knowledge. Among the leading physicists of the nineteenth century, it was Kirchhoff whose temperament was best suited to this task. In all his work he strove for clarity and rigour in the quantitative statement of experience, using a direct and straightforward approach and simple ideas. His mode of thinking is as conspicuous in his contributions of immediate practical value as in those with wide implications."
  
  L. Rosenfeld: "The excellence of Kirchhoff as a teacher can be inferred from the printed text of his lectures. They set a standard for the teaching of classical theoretical physics in German universities, at a time when they were taking a leading position in the development of science."

Connections

In 1857 Kirchhoff married Clara Richelot. She was the daughter of Friedrich Richelot, one of his mathematics professors from Konigsberg. In 1869 his wife died. He later married Luise Brommel of Goslar, the superintendent in the ophthalmological clinic, in Heidelberg in 1872.

Father:

Friedrich Kirchhoff

Mother:

Johanna Henriette Wittke

Spouse:

Luise Brommel

late spouse:

Clara Richelot

Son:

Paul Friedrich Robert Kirchhoff

Son:

Carl Gustav Ernst Kirchhoff

Daughter:

Julia Clara Paulina Kirchhoff

Son:

Friedrich Adolf Kirchhoff

Daughter:

Eveline Anna Kirchhoff

mentor:

Franz Ernst Neumann

colleague:

Robert Bunsen

colleague:

Henry Roscoe

Student:

Roland Eötvös

Student:

Edward Nichols

Student:

Gabriel Lippmann

Student:

Dmitri Mendeleev

Student:

Max Planck

Student:

Jules Piccard

Student:

Max Noether

Student:

Heike Kamerlingh Onnes

Student:

Ernst Schröder

References

• The Second Physicist: On the History of Theoretical Physics in Germany
  2017
• Dictionary of Scientific Biography, Volume 7
  1973