Popular Science Monthly/Volume 33/May 1888/Sketch of Gustav Robert Kirchhoff
THE history of physics in our century is not poor in eminent thinkers and great investigators; but it is safe to predict, as Prof. August Heller remarks, that when the student of a future age takes his perspective view of the achieved results of our contemporary research, he will pronounce Kirchhoff one of the greatest of them all. Yet, although his works have made his name immortal, and must cause it always to be in mind where physics is taught, so simple and modest was he as he is presented to us in Robert von Helmholtz's delineation of him, that his person is quite hidden behind the science to which he devoted his life; and that few, except fellow-laborers in the same lines and those who were so happy as to have had close relations with him, are aware of the extent and importance of his labors outside of the field of spectrum analysis.
Gustav Robert Kirchhoff was born—the son of counselor-at-law Kirchhoff—at Königsberg, Prussia, March 12, 1834. Having passed the course of the Kneiphof Gymnasium, he continued his studies at the Albertina in his native city, under Neumann in physics, and Julius Richelot in mathematics; and there, in his eighteenth year, decided that physics was the branch that pressed the strongest claims upon his attention. It was a period of rapid progress and important discoveries in science. Mayer had published his first paper concerning the forces of inanimate Nature, on the eve of the working out by several independent observers of the law of correlation and conservation; the undulatory theory of light had been established, but its mathematical conditions and its adjustment to facts remained to be worked out; and the wonderful properties and powers of electricity were under investigation by students at different centers, whose names have since become identified with various aspects of electrical theory. Kirchhoff, now entering upon the study of these same and related branches, was one of those fortunate young men, says Prof. Heller, who appear, by the nature of their faculties, to be specially adapted to their calling. His rare mathematical talent adapted him to the use of analytical aids to such an extent that he could always readily bring the best methods to the solution of any problem. On September 4, 1847, he took his degree from the University of Königsberg. In the following spring he began his professional career at the University of Berlin as a Privat-Docent. He had already, while a student, in 1845, published a paper in Poggendorff's “Annals” on electric conduction in a thin plate, and specially a circular one, to which were appended two theorems which have since become generally known as Kirchhoff 's laws. This was followed by other valuable papers on electrical questions, among which were those on conduction in curved sheets, on Ohm's law, on the distribution of electricity on two influencing spheres, on the discharge of the Leyden jar, on the motion of electricity in submarine cables, etc. Among them also is a paper on the determination of the constant on which depends the intensity of induced currents, in which is involved the absolute measurement of electric resistance in a definite wire.
In 1850 he was appointed Extraordinary Professor and Co-director of the Physical Institute in Breslau, where he remained four years, and formed a life-long fellowship and scientific brotherhood with Bunsen. In 1854, Bunsen having preceded him thither, he removed to Heidelberg, where he had been chosen regular Professor of Physics, in place of Jolly, who had been transferred to Munich. Here he lived and taught for twenty years, the bloom-period of his life. The brightest days in the history of this great university, to whose fame and pre-eminence Kirchhoff contributed very materially, fell during the same period. To the general public, says Robert von Helmholtz, hardly anything was then known of Kirchhoff. His labors at Berlin and Breslau, being in a field wholly theoretical, had attracted the attention only of experts. "There was, therefore, some surprise in Heidelberg when the slender, remarkably youthful, modest, even bashful North German appeared, heralded by Bunsen's warm recommendations. His refined, animated speech, his courteous and attractive demeanor, his fine sense of humor and his wit, soon won him the liking of all men with whom he came in contact. He was, therefore, a welcome participant in all the social gatherings of the circle into which he fell. His friendship with Bunsen became very close. Bunsen was thirteen years his elder, strong and broad-shouldered, with a lively, commanding temperament, making his influence felt upon every one. The two men were thus quite different in their outer aspects from one another; yet they not only pursued their great works in common, but also lived their daily social life together. They took walks in company in the environs of Heidelberg, and they traveled together during the vacations.
The discovery of the spectrum analysis is destined, like that of gravitation by Newton, and a few others, always to rank among the greatest achievements in the history of science. Newton had succeeded in separating white sunlight into its colored constituents. Wollaston had, in 1802, discovered the dark lines in the spectrum; and Joseph Fraunhofer had, independently, some ten years later, investigated those lines thoroughly, fixed the position of more than five hundred of them, and marked the principal groups with letters. Now, half a century afterward, Kirchhoff found the key to the remarkable phenomenon. For it is really the law of the relation of emission and absorption, as discovered by Kirchhoff, that furnishes the theoretical basis of spectrum analysis. More precisely expressed, this law declares that, for a given temperature and rays of the same color and polarity, the relation of the power of emission and absorption is the same for every body—that is, independent of the nature of the body. From this theorem it follows that a luminous body which sends out light-rays only of a certain wave-length, will also absorb rays only of the same wave-length. Under this law conclusions can be drawn from the dark lines of the solar spectrum concerning the constitution of the sun's atmosphere. Kirchhoff's first publication on this subject appeared in the monthly reports of the Berlin Academy for October, 1859. This short notice was followed by a rapid succession of papers describing the researches of the two investigators (Kirchhoff and Bunsen) upon the solar spectrum and the spectra of the elements.
While engaged in these investigations, Kirchhoff injured his vision by exposure to the glare of a clear spot in the solar spectrum, so that in later years he was obliged to spare his eyes. In 1867 he suffered an injury of his foot, in consequence of which he was for three years unable to get about except upon a perambulator or with the aid of crutches, and his health was affected for the remainder of his life. But the results of this personal mishap were seen in his physical life only, not in his labors.
In 1875 Kirchhoff accepted a call to the professorship of Mathematical Physics in the University of Berlin, after having previously declined two invitations to the same institution. Here he delivered for several years regular courses of lectures on the mechanics of solid and fluid bodies, the theory of heat and light, electricity and magnetism, mathematical optics, and special topics in hydrodynamics, electrodynamics, etc. “Whether life in Berlin,” says Robert von Helmholtz, “is favorable to scientific pursuits may well be doubted. The teacher, it is true, gains a wider, richer field of activity, but the investigator is robbed of a larger part of his time. Kirchhoff was, however, protected by his physical disability against most of the drive of the capital, and was able to labor as he had usually done. . . . His favorite work, and the one having the most enduring results, was his lectures on mathematical physics. His address was impressive by reason of the elegance and precision of his statement. Not a word was wanting, not a word was in excess; never an error, an obscurity, or an ambiguity. Remarkable also was the exactness of his calculations—a matter of extreme difficulty to laymen. The whole material arranged itself before the eyes of the class in the form of a nicely adjusted master-work of scientific art, so that every part exerted its full effect on the others, and to witness one of his deductions was a real aesthetic enjoyment. The complete understanding of his reasoning on these most difficult subjects implied, of course, some knowledge of the mathematical language which was his vehicle of thought; and it might happen, and did in fact sometimes happen, that a hearer could not comprehend why Kirchhoff made this particular deduction and not some other; but every one was able to follow his course of thought, consider it, and render it correctly. So that, paradoxical as it may appear, it was not impossible, without having really understood Kirchhoff, to reproduce his lectures from the notes into a respectable book. Kirchhoff was able to give his lectures uninterruptedly in Berlin for nine years. But we who heard him could remark the effort they caused him, and how he had to husband his strength. Yet he was always punctual, and the quality of his teachings was never depreciated. Finally, in 1884, the doctors forbade him to read; and although he was enabled to resume this his favorite occupation for a time, it was evident that his nervous system was shattered."
Besides the subjects we have already mentioned, Kirchhoff conducted a series of valuable investigations in the equilibrium and motion of elastic solids, especially in the form of plates and rods. His publications were not voluminous. His contributions to the Berlin Academy of Sciences are spoken of as having been about one a year. His collected papers (Gesammelte Abhandlungen), about fifty in number, were published in Leipsic in 1882, in a single volume. His lectures on dynamics (Vorlesungen über mathematische Physik), first published in 1876, have reached a third edition, at least. They are styled by Prof. Tait somewhat tough reading, but certainly recompensing the labor of following them. They form rather a collection of short treatises on special branches of the subject, than a systematic digest of it. His greatest work, "The Researches on the Solar Spectrum" (Untersuchungen über das Sonnenspectrum), was, almost immediately after its appearance, republished in an English translation. To these works, and the papers on radiation, partly mathematical and partly experimental, published in 1859 and 1860, which led up to the great work on the solar spectrum, he has added, so far as Prof. Tait has been able to discover, only three or four more recent papers, among which are one on the change of form which an elastic solid undergoes when it is magnetically or electrically polarized (Berlin Abhandlungen, 1884); a subsequent paper giving applications of the results in the same investigation; and additions to his paper on the distribution of electricity on two influencing spheres.
Prof. Heller says that Kirchhoff possessed in an eminent degree all the qualities most sought for in an academic teacher. Mr. Helmholtz sees in him the prototype of a genuine German investigator. The religion and object of his life was to seek the truth in its purest form, and express it with quite abstract unselfishness. He loved and cultivated science for itself alone, and deemed the slightest adornment or excursion from logical exactness in presenting it to be a profanation; while all mingling of it with personal motives or with the strife for honors or gain was most repugnant to him. As he acted in science, so did he in life; and what he recognized as a manly civic or official duty he pursued with logical thoroughness, divested of all personal motive. Winning amiability and goodness of heart were revealed in all of his personal intercourse, so that both in Heidelberg and Berlin he was one of the most popular of the academical teachers. He was fond of telling a story of how, when the conversation turned upon the question whether the Fraunhofer lines conveyed any information respecting the presence of gold in the sun, his banker asked, "Of what use is gold in the sun to me if I can not go and get it?" Afterward, having received an English gold medal for his discovery, he showed it to the banker, and said, "See, I have got some gold from the sun!" Having been compelled by his growing disabilities to retire from active life, Kirchhoff spent his last months with his family, preserving a living interest in the questions with which he had been occupied. He was never heard to utter a complaint, though he must have known that his powers were steadily passing away. Death came to him quite unexpectedly, while he was asleep, on October 17, 1887.
As described by Prof. Heller, he was of a stature rather under than above the average, with finely modeled, sharply cut features; having a high forehead, on which many years of continuous thought had engraved close and deeply cut wrinkles, while the penetrating glance of his deep-blue eyes bore witness to his habit of giving close attention to abstract thought.