WILHELM CONRAD RÖNTGEN.

A. F. Ioffe.

On February 11, 1923, at the age of 78, one of the most eminent physicists—W. C. Röntgen—died in Munich. He was born in 1845 in Germany, near the Dutch border, in the town of Lennep. At first he was preparing for the profession of engineer and studied at a polytechnic institute, but an interest in pure physics, already clearly manifested while he was still a student, led him to the university. After defending his doctoral dissertation he became an assistant in the department of physics, first in Zurich, then in Giessen, whence, together with his professor Kundt, he moved to Strasbourg. As an associate professor he was first in Giessen, and then again in Strasbourg. From there he went to a full professorship in Giessen, then to Würzburg and, finally, in 1900 to Munich. In 1919, on reaching the prescribed age limit, he handed over his chair to W. Wien, but retained the directorship of the Metronomic Institute in the same building of the Physics Institute of the University of Munich. Here he continued to work until the very last days of his life.

Röntgen received his training in physics from Kundt, who created a whole galaxy of major experimentalists (among them were the “Russian Kundt”—Petr Nikolaevich Lebedev and Aleksandr Aleksandrovich Eichenwald). He was also close to other outstanding contemporaries: Helmholtz, Kirchhoff, Lorentz; but with the years he increasingly withdrew into himself, and his contact with other physicists was limited to purely professional and scientific relations. He did not attend congresses of natural scientists, and in his private life and during his travels he did not go beyond the circle of his closest assistants and a few old friends—mathematicians, philosophers, physicians. Therefore his personal influence on contemporary physicists who had not been his pupils was slight. He enjoyed the reputation of the finest experimentalist: when Drude died, he was elected to the chair of physics at the University of Berlin; after Kohlrausch’s retirement he was offered—

post of president of the Physikalisch Technische Reichsanstalt and, after the death of Van ’t Hoff, the place of academician. However, he declined all these offers, just as he did the offers of various orders (including Russian ones) that followed upon his discovery, and until the last years of his life he called the rays X-rays.

Among his students many occupy chairs, and in all their scientific activity the traces of Röntgen’s school are visible. Such are: M. Wien, L. Zehnder, P. P. Koch, E. Wagner, A. Schmauss, R. Ladenburg, P. Pringsheim, E. Angerer, Valentiner, Friedrich, du Prel, and others.

In the course of his almost fifty-year scientific activity Röntgen published altogether about 25 works, devoted to several questions. The best known of them are the X-rays he discovered and the Röntgen current (the magnetic field of a dielectric moving in an electric field). A large number of his works are devoted to the properties of liquids (compressibility, internal friction, surface tension), gases (absorption of infrared rays, measurement of \(\dfrac{C_p}{C_v}\)), crystals (piezo- and pyroelectricity, electro- and piezo-optical properties), and electro-optical phenomena (double refraction in liquid and crystalline bodies in an electric field, ionization of crystals by light). Each work was printed only when Röntgen considered its results entirely beyond doubt. A large number of other experiments carried out by him were not published, since uncertainties still remained in them.

Brilliant experimental talent, a clear and simple formulation of the experiment, a comprehensive and subtle analysis of possible errors, the highest precision and reliability of the results obtained—these are the features common to all his 25 works, which made them classical. The greater part of his work is of a measuring character. Röntgen always achieved greater accuracy than others, and many of his measurements remained records even after 40 years \(\left(\dfrac{C_p}{C_v},\right.\) compressibility, and others\(\left.\right)\). However, he achieved this accuracy not by complicating the apparatus and introducing numerous corrections (as Regnault, for example, did), but by applying a new, purposefully devised method, which radically eliminated the most important errors and made it possible to attain accuracy with the help of simple, often homemade instruments, corresponding to his taste, as he himself expressed it in one of his notes. Thus, for example, for many years there was a dispute between two of the greatest physicists, Tyndall and Magnus, as to whether water vapor absorbs infrared rays. Tyndall’s experiments gave an affirmative answer; Magnus always found errors in them and denied the absorption. The density of the vapor is small; it was necessary to know very precisely the quantity of rays entering the vapor and leaving—

...emerging from it, in order to measure the small difference absorbed by the vapor. One might even climb Mont Blanc in order to determine the absorption in the layer of air between its summit and its foot. Röntgen, having posed this question to himself, proceeded more simply: he measured the increase in pressure that must occur in a closed vessel with water vapor or another gas as a result of heating by the absorbed rays. Without complicated instruments or grandiose schemes he succeeded not only in establishing the fact of absorption, but also in measuring it quantitatively.

Equally characteristic is his work on the Röntgen current. A dielectric rotates between the charged plates of a capacitor and creates a magnetic field. A doubt might arise: does the current not flow along the metallic plates of the capacitor themselves, and is it not this current that produces the magnetic effect? Röntgen dispels this doubt by a simple experiment: the plates are made of metallic sectors separated by ebonite gaskets; there can be no current in the plates, while Röntgen’s effect remains unchanged.

It is curious, perhaps, to note that the famous method of crossed spectra of Kundt, who discovered anomalous dispersion, was prompted by Röntgen’s criticism. The latter, being color-blind, distinguished red and blue colors poorly and therefore was not at all convinced by measuring the order of colors in the spectrum. He demanded from Kundt more objective proof, not dependent on the eyesight of one physicist or another, and as a result of a detailed analysis of the possibilities Kundt devised his method.

The most important of Röntgen’s works are, undoubtedly, the three papers “On a New Kind of Rays,” printed in the years 1895–97. They won him worldwide fame, brought him the Nobel Prize, and exerted the most decisive influence on the entire subsequent development of physics. The discovery of Röntgen rays may be regarded to some extent as accidental, although, all the same, extraordinary powers of observation were required in order, barely having begun the study of cathode rays, immediately to notice the new Röntgen rays created by them. Tubes with cathode rays had existed for 40 years, but none of those who worked with them (including Lenard, who believed that he should, “in fairness,” have discovered these rays) noticed them. But in any case it is not accidental, but is most intimately connected with Röntgen’s entire scientific personality, the form into which his investigation took shape. In three small papers, published over the course of one year, such an exhaustive description of the properties of these rays was given that hundreds of works that followed over the next 15 years could neither add nor change anything essential. And all this investigation was carried out in a completely new field with the most elementary means: the only “instrument” that Röntgen used here was an electroscope with leaves. To study each property of the rays he devised...

were new, extraordinarily ingenious methods, subsequently used more than once in the most varied cases.

Let us recall some of Röntgen’s devices of this kind:

1) Reflection. Röntgen observed that the rays are not noticeably reflected even from well-polished surfaces. There remained, however, the possibility that, in comparison with light, the difference was only quantitative: the coefficient of reflection of X-rays is very small. Instead, however, of measuring this small quantity by improving the measuring instruments, Röntgen establishes that a substance pounded into powder and an integral substance are equally transparent to X-rays; hence it follows that the numerous surfaces of the separate grains of the powdered body do not reflect and do not scatter the rays more than the interior of the whole body. Röntgen gives a perfectly precise description of the scattering and absorption of the rays, comparing the body with a room full of tobacco smoke, through which a ray of light passes. Each atom inside the body and on its surface scatters equally, and the more strongly the greater its atomic weight. Röntgen raises the question whether the scattered rays are identical with the primary ones, and quite correctly assumes that, alongside the deflected primary rays, there appear also other, always softer rays, created by the atoms of the scattering body. The very characterization of the hardness of rays by their absorption, which survived even after Laue’s discovery alongside quantitative spectroscopy, belongs to Röntgen.

2) Ionization. Röntgen observes the discharge of an electrified body under the influence of the rays and at once establishes that the principal role is played by the ionization of the air. Rays passing near an electrified body discharge it in the same way as rays falling directly upon it. However, this effect too can be attributed to secondary rays produced in the air and reaching the body. Röntgen shows that if air illuminated by the rays is drawn through a long tube, it retains the ability to discharge a charged body. By placing a cotton plug in the path of the ionized air in the tube, one can deprive the air of its ability to remove charges from bodies. In order to make sure that the cause of this phenomenon lies in the contact of the ionized air with the surfaces of the pores in the cotton, and not in the slowing down of the air moving in the tube, Röntgen places the same plug at a point in the tube through which the air passes even before ionization (on the other side of the section of the tube illuminated by the rays). The motion of the air in the tube is slowed equally wherever the plug is placed, whereas the discharging ability is preserved if the ions have not come into contact with the cotton.

3) The very first experiments with the rays lead Röntgen to the correct construction of the tube—the inclined platinum anticathode, the concave cathode. The photographs made by him at that time are examples—

experimental art; thus, for example, he obtained an image of an inscription engraved on the barrel of a hunting gun.

4) Roentgen’s extraordinary experimental instinct is attested by his persistent attempts to detect the effect discovered by Laue 17 years later. Having established that rays are scattered by every atom, Roentgen concludes that, with the proper arrangement of atoms present in a crystal, scattering and absorption should depend on direction. He seeks this phenomenon in a setup very much resembling the experiments of Laue and Friedrich, but only with a photographic plate pressed against the crystal. He could not have had more subtle considerations about diffraction or interference, since the wave nature of the rays was not known. Yet Roentgen’s basic considerations were so convincing that in each of three papers he repeats his confidence in the existence of the effect, despite the fact that all his attempts yielded a negative result. Even if the chance that had so favored him in the discovery of the rays had caused Roentgen to place the photographic plate in the right position, then, given the low power of the tubes of that time, he could hardly have detected the sought-for effect. Indeed, the first experiments of Friedrich, who knew what he was looking for, also gave a negative result, and only the photographic plate, placed at random by Knipping in the path of the rays, led to Laue’s discovery. In 1895 and 1896 there was not yet a basis for the new discovery, but Roentgen knew where to look for it. Roentgen’s hypothesis concerning the physical nature of his rays—as longitudinal vibrations of the ether—was also not justified; nevertheless, taking into account the origin of the rays under the longitudinal impact of the cathode stream and their sharp difference from light rays, Roentgen’s hypothesis cannot but be regarded as quite natural for that time.

Alongside these examples of qualitative research, Roentgen also knew how to carry out the most exact measurements wherever this was required by the formulation of the problem. Thus, for example, he saw that it was impossible to create a theory of the liquid state without having a complete quantitative characterization of the properties of various liquids and solutions. And he performed this work with astonishing mastery and consistency, spending ten years on it (from 1883 to 1892). Measurements of the constants of water and aqueous solutions led him in 1892 to the idea that water is an equilibrium system of molecules of different composition. The question of the relation between pyroelectricity and piezoelectricity received the same quantitative character. The works of 1913–1914 provided exhaustive material, reducing pyroelectricity without remainder to piezoelectricity. To this same group belong the classical experiments on the determination of \(\frac{C_p}{C_v}\) with the aid of a small mirror successfully placed on a rod,

measurement of the thermal expansion of diamond at low temperatures, which led to Debye’s theory.

In all his views and activity, Röntgen was a typical representative of the classical physics of the second half of the last century. To the same school belonged Kundt, Warburg, Kohlrausch, Rubens, Braun, Paschen, Curie—almost all of them now departed from us. Röntgen, more than any of his contemporaries, contributed to the creation of the new physics of our century—the physics of elementary processes and electronic phenomena. Nevertheless, he himself remained faithful to the old precepts and kept aloof from that flood of not always sufficiently substantiated “discoveries” and hypotheses which followed upon his discovery. Here one could see his organic aversion to the publication of unfinished, superficial generalizations, of concepts behind which there was no strictly formulated and experimentally proved content. He found justification for his conservatism in the short-lived existence of various kinds of “new rays”—of Gretz, Blondlot, and others—new words with which people tried to replace the explanation of phenomena.

Here is a characteristic example of how Röntgen kept aloof from those fields in which, following his example, young and old physicists were gaining easy fame: the subject of my doctoral dissertation was the study of the elastic after-effect in quartz. The original task was to decide whether the direct cause of piezoelectricity was the deformation of quartz or the stress existing in it (and in the ether). In the after-effect only one of these quantities changes, while the other remains constant, and, consequently, it was possible to separate their influence. It seemed to me, however, that elastic after-effect in quartz did not exist at all, and that the observed phenomena were caused by piezoelectric charges in the mass of the crystal. To eliminate them, I tried to increase the electrical conductivity of quartz by using Röntgen rays, radium, ultraviolet light, etc., and indeed found a sharp increase in electrical conductivity. Investigating in this direction a number of other crystals and dielectrics, I found a number of very curious new facts and regularities, about which I hastened to inform Röntgen, who was then in Santa Margareta. In reply to this I received from him the following admonition: “I expect from you solid scientific work, not sensational discoveries.” On returning to Munich he expounded to me in detail his negative view of work of this kind and warned me against beginning my scientific activity with radium and Röntgen rays, where every piece of work is drowned in a mass of other unverified and unreliable facts. And only when, continuing my experiments despite the warning, I noticed the enormous influence of ordinary light on the electrical conductivity of rock salt, did Röntgen, seeing here

new kind of connection between light and electricity, became interested in the phenomenon; from then on we carried on this work together.

Another very striking example is Röntgen’s negative attitude toward the electron, which continued until 1906–1907 and went so far that in the Physical Institute of the University of Munich the word electron was not uttered. Röntgen considered its existence unproven, leading to internal contradictions (Michelson’s experiment) and being applied too incautiously even where it explained nothing. For two years, in my daily conversations with Röntgen, I made use of this concept in order to show the usefulness and even the inevitability of its recognition, until finally Röntgen acknowledged that the totality of accumulated experimental data was sufficient to substantiate the electron.

Röntgen attached objective significance to facts, not to their explanation. Therefore, despite the fact that we had succeeded in fully clarifying certain aspects of the electrical conductivity of crystals (Ohm’s law, polarization, electrolysis), Röntgen considered it necessary, without giving the explanations we had found, first to set forth and systematize the experiments objectively, so that only in the conclusion their meaning might be indicated. As a result of prolonged disputes, Röntgen admitted the usefulness of another method of presentation, in which the picture of the phenomenon would already be clear to the reader studying its concrete foundation. However, this method of presentation so contradicted Röntgen’s scientific habits and principles that only with extraordinary slowness did he prepare our work for print and, having begun in 1907, only in 1913 did he publish the first part, with a description of the apparatus and methods of observation; in 1921—the second, with a purely objective description of the phenomena accompanying the passage of current through rock salt; and only in the summer of 1922 did he ask me to write the main part as I considered necessary; this part will appear only after his death, although he still managed to acquaint himself with it and discuss it.

Röntgen highly valued the best representatives of the “new physics”: J. J. Thomson, Rutherford, Millikan, Sommerfeld, Einstein, and Bohr (the last he even proposed as a candidate for the Nobel Prize), but he himself kept aloof from it. And the more hasty “preliminary communications” appeared in the German scientific literature, the more solid and documentary his own works became; the last of his works, with its own experimental material, occupied an entire booklet of Annalen der Physik of 200 pages.

Röntgen remained just as consistent, faithful to principles once and for all worked out, in private life as well. He was no diplomat; he could not adapt himself either to those below or to those above. In the faculty and in the academy he followed his own line, without regard for any influences. When, after the refusal of Lorentz, he considered it necessary

A. F. IOFFE

to propose to Sommerfeld the chair of theoretical physics at the University of Munich, which indeed raised it to a great height, Röntgen was not afraid to enter into the fiercest dispute with the influential group of the mathematician Lindemann. He had to do a great deal of work in order to refute all of Lindemann’s objections to one of Sommerfeld’s principal works, and to secure his election. Röntgen allowed no deviations from the method of work which he considered the only scientific one, either for any of his assistants and pupils or for those in authority. He was just as consistent with those in power: when Wilhelm II, during a visit to the German Museum in Munich, after hearing Röntgen’s explanations of the physics section, tried likewise to explain artillery to Röntgen and could say nothing except well-known trivial phrases, Röntgen told him so directly; after which Wilhelm, turning away, immediately left, offended in his pride as a specialist.

In 1917, as a result of the blockade, famine reigned in Germany, and the entire population received the meager foodstuffs distributed equally by ration cards. Röntgen had many friends in Holland who sent him food parcels with butter and sugar. However, Röntgen believed that in time of famine no one should enjoy privileges, and he turned over all his parcels to the state for general distribution. In that year he lost 1½ poods in weight, and only when the doctors declared that another month of such a life would lead him to death did he agree to accept an increased hospital ration.

During the war, especially for the purchase of food, the state needed foreign currency. All of Röntgen’s considerable capital had been invested in Dutch securities, and he gave it all, without remainder, at the first request. Here too he knew no compromises, and from many hundreds of thousands he did not leave himself a single guilder. In the last years of his life he was forced to deny himself the most necessary things. Only once a week could he allow himself a meat dish. In order to fulfill his wish, before his death, to visit once more the places in Switzerland where he had lived with his recently deceased wife, he had for an entire year to give up coffee, etc.

Röntgen was a great and integral man—in science and in life. His entire personality and his activity belong to the past. But only because Röntgen lived could modern physics appear. Röntgen’s current was the impetus to electronic theory; X-rays—to modern electronics. Upon the firm foundation created by him a new building grew. If the bright coloring of certain details often contradicted his taste, still the foundation, the material, and the methods for the construction were given to us by Röntgen.

List of the Scientific Works of W. C. Röntgen

1. Report of some experiments in the field of capillarity. Wied. Ann. 1878. 3. 321—328.

2. On an aneroid barometer with mirror reading. Wied. Ann. 1878. 4. 305—311.

3. On the new relation between light and electricity found by Mr. Kerr. Wied. Ann. 1880. 10. 77—92.

4. On tones that arise through intermittent irradiation of a gas. Wied. Ann. 1881. 12. 155—159.

5. On the change in the double refraction of quartz produced by electric forces. Wied. Ann. 1883. 18. 213—228. Ber. d. Oberhess. Ges. f. Natur u. Heilkunde, 1882. 22. 49. Wied. Ann. 1883. 19. 319—513.

6. On a lecture apparatus for demonstrating Poiseuille’s law. Wied. Ann. 1883. 20. 268—271.

7. On the influence of pressure on the viscosity of liquids, especially water. Wied. Ann. 1884. 22. 510—518.

8. New experiments on the absorption of heat by water vapor. Wied. Ann. 1884. 23. 1—49, 259—298.

9. On the compressibility and surface tension of liquids. Wied. Ann. 1886. 29. 165—213.

10. On the compressibility of dilute salt solutions and the compressibility of solid sodium chloride. Wied. Ann. 1887. 31. 1000—1006.

11. On the compressibility of water. Wied. Ann. 1888. 33. 644—661.

12. Electrical properties of quartz. Wied. Ann. 1890. 39. 16—24.

13. On the compressibility of carbon disulfide, benzene, ethyl ether, and some alcohols. Wied. Ann. 1891. 44. 1—23.

14. On the influence of pressure on the refractive exponents of water, carbon disulfide, benzene, ethyl ether, and some alcohols. Wied. Ann. 1891. 44. 24—51.

15. On the constitution of liquid water. Wied. Ann. 1892. 45. 91—97.

16. Short report of experiments on the influence of pressure on some physical phenomena. Wied. Ann. 1892. 45. 98—107.

17. On the influence of the heat of compressibility on the determination of the compressibility of liquids. Wied. Ann. 1892. 45. 560—567.

18. Method for producing clean water and mercury surfaces. Wied. Ann. 1892. 46. 152—157.

19. Note on a method for measuring pressure differences by means of mirror reading. Wied. Ann. 1894. 51. 414.

20. Report of some experiments with a right-angled glass prism. Wied. Ann. 1894. 52. 589—592.

1. Ueber den Einfluss des Druckes auf die Dielektricität des Wassers und des Aethylalkoholes. Wied. Ann. 1894. 52. 593—603.

2. Ueber eine Neue Art von Strahlen. Ber. d. Würzb. Ges. Dez. 1895. März 1896. März 1879. Ann. d. Phys. 1898. 1. 1—37.

3. Elektricitätsleitung in Kalkspath. Ber. d. Münch. Akad. 1907,

4. Ueber die Wärmeausdehnung des Diamanten. Ber. d. Münch. Akad. 1912.

5. Ueber die Elektricitätsleitung in Kristallen und über den Einfluss einer Bestrahlung darauf. Ann. d. Phys. 1913, 1921, 1923.

6. Pyro-und Piezoelektrische Untersuchungen. Ann. d. Phys. 1914. 737—800.