Philipp Lenard


Philipp Lenard

Philipp von Lenard was born at Pozsony1 (Pressburg) in Austria-Hungary on June 7, 1862. His family had originally come from the Tyrol. He studied physics successively at Budapest, Vienna, Berlin and Heidelberg under Bunsen, Helmholtz, Königsberger and Quincke and in 1886 took his Ph.D. at Heidelberg.

From 1892 he worked as a Privatdozent and assistant to Professor Hertz at the University of Bonn and in 1894 was appointed Professor Extraordinary at the University of Breslau. In 1895 he became Professor of Physics at Aix-la-Chapelle and in 1896 Professor of Theoretical Physics at the University of Heidelberg. In 1898 he was appointed Professor Ordinarius at the University of Kiel.

Lenard’s first work was done in the field of mechanics, when he published a paper on the oscillation of precipitated water drops and allied problems and in 1894 he published the Principles of Mechanics left behind by Hertz.

Soon he became interested in the phenomena of phosphorescence and luminescence. This was a development of the mysterious attraction which weak light appearing in darkness had had for him since his boyhood, when he had, with his school fellows, warmed fluorine crystals to make them luminescent; and now he took up, with the astronomer W. Wolf, the study of the luminosity of pyrogallic acid when it is mixed with alkali and bisulphite for developing photographs. He found that its luminosity depended on the oxidation of the pyrogallic acid. At this time he also carried out studies of magnetism with bismuth and, in collaboration with V. Klatt, who had been his first teacher of physics in his native town, he studied, at the Modern College at Pressburg, the so-called self-luminous substances such as calcium sulphide on which Klatt had been working for some years. Together they found that calcium sulphide, after previous illumination, exerts light in the dark, but only if it contains at least some traces of heavy metals, such as copper and bismuth, which form crystals on which the colour and the intensity and durations of the luminosity depend; if it is quite pure, it is not luminous. This work with Klatt was the beginning of work in a field which occupied Lenard for the next 18 years.

In 1888, when he was working at Heidelberg under Quincke, Lenard had done his first work with cathode rays. He investigated the view then held by Hertz that these rays were analogous to ultraviolet light and he did an experiment to find out whether cathode rays would, like ultraviolet light, pass through a quartz window in the wall of a discharge tube. He found that they would not do this; but later, in 1892, when he was working as an assistant to Hertz at the University of Bonn, Hertz called him to see the discovery he had made that a piece of uranium glass covered with aluminium foil and put inside the discharge tube became luminous beneath the aluminium foil when the cathode rays struck it. Hertz then suggested that it would be possible to separate, by means of a thin plate of aluminium, two spaces, one in which the cathode rays were produced in the ordinary way and the other in which one could observe them in a pure state, which had never been done. Hertz was too busy to do this and gave Lenard permission to do it and it was then that he made the great discovery of the “Lenard window”.

After many experiments with aluminium foil of various thicknesses he was able to publish, in 1894, his great discovery that the plate of quartz that had, until then, been used to close the discharge tube, could be replaced by a thin plate of aluminium foil just thick enough to maintain the vacuum inside the tube, but yet thin enough to allow the cathode rays to pass out. It thus became possible to study the cathode rays, and also the fluorescence they caused, outside the discharge tube and Lenard concluded from the experiments that he then did that the cathode rays were propagated through the air for distances of the order of a decimetre and that they travel in a vacuum for several metres without being weakened. Although Lenard at first followed Hertz in believing that the cathode rays were propagated in the ether, he later abandoned this view as a result of the work of Jean Perrin in 1895, Sir J.J. Thomson in 1897 and W. Wien in 1897, which proved the corpuscular nature of the cathode rays.

Later Lenard extended the work of Hertz on the photoelectric effect. Working in a high vacuum, he analysed the nature of this effect, showing that when ultraviolet light falls on a metal it takes from the metal electrons which are then propagated in the vacuum, in which they can be accelerated or retarded by an electric field, or their paths can be curved by a magnetic field. By exact measurements he showed that the number of electrons projected is proportional to the energy carried by the incident light, whilst their speed, that is to say, their kinetic energy, is quite independent of this number and varies only with the wavelength and increases when this diminishes.

These facts conflicted with current theory and were not explained until 1905, when Einstein produced his quantitative law and developed the theory of quanta of light or photons, which was verified much later by Millikan. But Lenard never forgave Einstein for discovering and attaching his own name to this law.

In the course of his work Lenard had, for the purpose of accelerating the speed of the electrons and measuring their energy, invented a photoelectric cell which was the first model of the “3-electrode lamp” which is so important today in radioelectric technique. The only difference between these two cells was that in Lenard’s cell the electrons were taken from the cathode by light, whereas on the “3-electrode lamp” the cathode is a white-hot filament capable of sending into the vacuum currents of much higher intensity.

In 1902 Lenard showed that an electron must have a certain minimum energy before it could produce ionisation when it passed through a gas.

In 1903 he published his conception of the atom as an assemblage of what he called “dynamides”, which were very small and were separated by wide spaces; they had mass and were imagined as electric dipoles connected by two equal charges of contrary sign and their number was equal to the atomic mass. The solid matter in the atom was, he thought, about one thousand millionth of the whole atom. This work contributed much to Lorentz‘ theory of electrons.

In his later years Lenard studied the nature and origin of the lines of the spectrum. Developing the work of Rydberg, Kayser and Runge, who had shown that the lines of the spectrum of a metal can be arranged in two or more different series and that there is a marked mathematical relationship between the wavelengths of these series, Lenard showed that in each series a definite modification of the atom has occurred and that these modifications determine the series and are differentiated by the number of electrons lost.

Lenard was an experimentalist of genius, but more doubtful as a theorist. Some of his discoveries were great ones and others were very important, but he claimed for them more than their true value. Although he was given many honours (for instance, he received Honorary Doctorates of the Universities of Christiania, now Oslo, in 1911, Dresden in 1922 and Pressburg in 1942, the Franklin Medal in 1905, the Eagle Shield of the German Reich in 1933, and was elected Freeman of Heidelberg in the same year), he believed that he was disregarded and this probably explains why he attacked other physicists in many countries. He became a convinced member of Hitler’s National Socialist Party and maintained unreserved adherence to it. The party responded by making him the Chief of Aryan or German Physics. Among his publications are several books: Ueber Aether und Materie (second edition 1911), Quantitatives über Kathodenstrahlen (1918), Ueber das Relativitätsprinzip (1918) and Grosse Naturforscher (second edition 1930).

Von Lenard, who was married to Katharina Schlehner, died on May 20, 1947 at Messelhausen.

1. Today Bratislava, located in Slovakia.

From Nobel Lectures, Physics 1901-1921, Elsevier Publishing Company, Amsterdam, 1967

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