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1. Biographical note *
Although no book-length biography of Pieter Zeeman exists, a number of short biographical sketches are available - all in Dutch as far as I know - which present the main facts of Zeeman's life. Usually, they also contain a more or less extensive overview of his scientific work. They are listed in the bibliography (p. 15). * This biographical note is based almost entirely on these already existing biographies, so as to leave the rich material contained in the Pieter Zeeman papers entirely to a future biographer. *
Pieter Zeeman was born on the 25th of May 1865 in Zonnemaire, in the province of Zeeland, in the Netherlands. He was the son of Catherinus Forandinus Zeeman (1828-1906), a minister of the Dutch Reformed Church and of Willemina Worst (1828-1907), whose father had also been a minister of that Church.
In 1868, a brother, Johannes Huibertus (Bert), was born, while Pieter and Bert had two more brothers and two sisters who died in infancy.
Pieter went to the primary school in Zonnemaire and from 1876 to 1882 he attended the 'Hogere Burger School' (HBS) * in Zierikzee. He passed his final examination in 1882 with an excellent mark for physics. Zeeman's physics teacher in Zierikzee, P. Schuringa, impressed by the boy's aptitude for the subject, advised his parents to allow him to study physics at the university. As a HBS graduate, however, Zeeman first had to pass a supplementary examination in Latin and Greek before he could be admitted to take academic examinations. To prepare himself for this supplementary examination, he moved to Delft to stay with the classicist J.W. Lely. However, he spent most of his time studying physics and astronomy. Through Lely, Zeeman met H. Kamerlingh Onnes, professor of experimental physics in Leiden. Kamerlingh Onnes later reported that, at that meeting, he became impressed by Zeeman's wide reading in the physical sciences and encouraged him to study physics. * In 1884 Zeeman failed at his first attempt to pass the supplementary examination but in 1885 he succeeded and enrolled at the University of Leiden in September of that year.
During the first years of his studies, before the 'kandidaats', the first academic examination, he took courses in a wide variety of subjects as the student notebooks (inv.nrs. 257-295) indicate. After the 'kandidaats', which he took in 1887, he specialized in experimental physics. The first research notebook (inv.nr. 498), beginning in 1887, shows that Zeeman assisted R. Sissingh in the investigation of the Kerr magneto-optic effect, the phenomenon that 'when plane-polarized light is reflected at normal incidence from the polished pole of an electromagnet, it becomes elliptically polarized to a slight degree'. *
After passing his 'doctoraal', the second academic examination, in 1890, Zeeman obtained an assistantship, a paid position in which he had to teach a few courses and prepare a dissertation. Judging from the research notebooks, he continued the investigation of the Kerr-effect along the same lines as before his 'doctoraal'. In 1892, he submitted part of his results as an answer to a prize question of the 'Hollandsche Maatschappij der Wetenschappen' * and won the Gold Medal.
On 18 January 1893 he obtained his doctorate after defending his dissertation * which consisted largely of the same material.
In February of that year, he left for Strasburg, to work with Emil Cohn on the propagation of electrical waves in liquids, returning to Leiden in August. After his return his academic career took off when he was appointed as 'privaat docent', a position in which he had to teach a few courses but could also devote time to research. He continued his investigations of the Kerr-effect and of the propagation of electrical waves.
Meanwhile, Zeeman had become engaged to Johanna Elisabeth Lebret (1873-1962), the sister of his student friend Adriaan Lebret and the daughter of Martinus Cornelis Lebret (1833-1894), a well-to-do merchant from Dordrecht, and Elisabeth Giltay (1845-1936). They were married on 28 March 1895.
In September 1896, while in the midst of his electrical wave research, Zeeman made an attempt to find out if magnetism influenced the emission of light. He had apparently tried to do so before, without result * . He was inspired to try again by the reading of J.C. Maxwell's biographical sketch of Michael Faraday * . Faraday had discovered the connection between light and magnetism, by observing that when plane-polarized light goes through transparent matter in a magnetic field, the plane of polarization is rotated. He had conjectured that there ought to be a corresponding effect in the emission of light. Towards the end of his life he had tried to find this effect but failed to detect it due to technical shortcomings in his equipment * . When Zeeman tried again in 1896 he was equipped with an excellent Rowland grating and a powerful magnet and he saw that the D lines of the spectrum emitted by natrium broadened to two or three times their size, when placed in a strong magnetic field. After several days of further investigation of this phenomenon and fearing that it was nothing more than an artefact, he continued his work on the electrical waves. He took up the investigation of the broadening of the spectral lines again in the beginning of October. For six weeks on end, Zeeman attempted to determine beyond any doubt that he had discovered a genuine effect * . On November 23, his doubts were removed when he observed that the edges of the broadened D lines were circularly polarized. This confirmed a theory proposed by H.A. Lorentz, professor of theoretical physics in Leiden. Lorentz had explained Zeeman's discovery on the basis of his electron theory and had predicted this circular polarization.
Meanwhile, Kamerlingh Onnes had reported on Zeeman's discovery at two monthly meetings of the 'Koninklijke Akademie van Wetenschappen' (October 31 and November 28, 1896) and soon translations of these 'Akademie' communications in English and German * spread the exciting news.
The importance of Zeeman's discovery and Lorentz's explanation was immediately recognized. Lorentz had formulated and repeatedly adjusted his electron theory in an attempt to integrate Maxwell's theory which described the interplay between electrical currents, electrical fields and magnetic fields into an atomistic description of matter. In his theory, Lorentz had assumed the existence of small particles within the atom which carry mass and electrical charge and which are responsible for the emission and absorption of light by matter.
Zeeman's discovery provided experimental proof for the existence of these particles, which were first called ions and later electrons. In 1902, both scientists received the Nobel Prize in Physics for this work.
Meanwhile, Zeeman continued investigating the phenomenon and before long he observed the broadening of a spectral line as an actual splitting up in several components: three, when looking perpendicularly to the lines of force of the magnetic field (transversal effect), and two when looking in a direction parallel to the lines of force (longitudinal effect). From a comparison between the frequencies of the right and the left circularly polarized rays, Zeeman concluded that the 'ion' had a negative electrical charge. From a measurement of the distance between the component spectral lines, Zeeman derived the relation between the e (the charge) and m (the mass) of the 'ion', which he calculated to be less than 1/1000 of the mass of an atom of hydrogen.
Soon Zeeman, and others, observed that spectral lines usually split up in more complicated patterns than triplets and doublets. Lorentz's electron theory did not provide an explanation of this phenomenon which was later called the anomalous Zeeman-effect. These complicated patterns would only be fully explained several decades later by the application of the quantum theory.
In 1897, Zeeman had been appointed as 'lector' * at the University of Amsterdam and he intended to use the 'Natuurkundig Laboratorium' there for his experiments. He very much wanted to continue to work on the anomalous Zeeman-effect, but the physics laboratory in Amsterdam was built on relatively soft ground which transmitted vibrations caused by traffic. Consequently, Zeeman had to abandon this line of research, which required absolute stability of the instruments. He continued his research in magneto-optics in general, and specifically in other aspects of the Zeeman-effect, until 1912. In close cooperation with Woldemar Voigt, a theoretical physicist from Göttingen, he successively investigated asymmetry in triplets, magnetic rotation of the plane of polarization and magnetic double refraction.
At the end of this period he studied the magnetic separation of sun spot spectra and soon afterwards he published two fairly extensive accounts on the state of magneto-optic research * . With these articles Zeeman's active concern with magneto-optics ends.
In 1900 Zeeman was promoted to the rank of extraordinary professor. When J.D. van der Waals sr. retired as professor of theoretical physics, in 1908, his son J.D. van der Waals jr. was appointed as his successor and Zeeman became professor of experimental physics. Other colleagues in the Physics Department were R. Sissingh, extraordinary professor since 1897 and ordinary professor since 1907, and Ph. Kohnstamm, extraordinary professor since 1908.
In 1908 Zeeman succeeded Van der Waals sr. as director of the 'Natuurkundig Laboratorium'. This physics laboratory dated from the 1880's and the lack of space and the susceptibility to vibrations made Zeeman feel the need for more modern surroundings. He immediately started lobbying for a new building. In 1914, he succeeded in obtaining permission from the municipal government of Amsterdam for the building of a new laboratory, but it took many years before construction was started. One of the obstacles was the high level of prices of building material because of the First World War. The new 'Laboratorium Physica' was finally opened on 18 June 1923 and Zeeman, of course, became its director. Sissingh succeeded him as director of the old 'Natuurkundig Laboratorium'. In 1940, on the occasion of Zeeman's 75th birthday, the 'Laboratorium Physica' was renamed 'Zeeman Laboratorium'.
Since his discovery Zeeman had become a much honored member of the scientific community. He received numerous awards, honorary doctorates, invitations to conferences and other signs of recognition. In addition, Zeeman turned out to be a skilful and well-appreciated member of boards and committees. * Special mention should be given to his tenure as general secretary and secretary of the Section of Sciences of the 'Koninklijke Akademie van Wetenschappen', from 1912 to 1921, his chairmanship of the Faculty of Mathematics and Physical Sciences, from 1913 to 1917, and his membership and vice-presidency of the 'Comité International des Poids et Mesures', from 1929 to 1943.
In 1912, Zeeman tackled a new research subject which dominated his scientific activities until he moved to his new laboratory in 1923: the experimental confirmation of Lorentz's correction of Fresnel's formula for the speed of light in moving, transparent media, which included the well-known 'dragging-coefficient'. Because of the ingenuity of the design of the apparatus and the accuracy of the measurements, he gained much admiration with these experiments. * Another example of Zeeman's experimental skills and his desire for exactitude can be found in the experiments he conducted in 1917 and 1918 in his countryhouse, 'Zonnehof', to determine the ratio of mass to weight, the equality of which is one of the cornerstones of the general theory of relativity. He succeeded in obtaining results many times more precise than those achieved by other researchers. *
Elsewhere, the investigation of magnetic resolution of spectral lines had meanwhile continued. Many data on the complex patterns of the anomalous Zeeman-effect in substances had been collected, in the hope of one day understanding the phenomenon.
A new impetus for the search for an explanation was the development of quantummechanics during the 1920's. The complex patterns were beginning to be understood on the basis of the quantum theory of atomic structure and radiation. The study of these patterns became a potent tool in the analysis of complex spectra. *
In the years before his retirement, Zeeman took an interest in nuclear physics. He investigated the characteristics of the nucleus of an atom through the observation of the hyperfine structure of spectral lines. In the early 1930's he developed a new tool, the parabole spectrogramme, with which he was able to prove the existence of a new isotope of argon. *
In 1935 Zeeman retired as professor at the age of 70. It took five years to appoint a successor to him. Zeeman had pronounced views on the kind of scientist he wanted to take his place, and, above all, he wished that the spectroscopy section would continue its operations unobstructed and under the direction of a true spectroscopist. For Zeeman the course of events concerning his succession was a bitter disappointment: his retirement had been the beginning of a fierce faculty power struggle, in which his suggestions for successors came to nothing, and the importance of the spectroscopy section was put into question. The result was that teaching and research activities in spectroscopy were, to Zeeman's regret, seriously reduced. Finally, in 1940, C.J. Gorter, a physicist from Leiden, was appointed as professor of experimental physics and director of the 'Zeeman Laboratorium'. Gorter was succeeded in 1946 by C.J. Bakker, one of Zeeman's former assistants. Also in 1940, T.L. de Bruin, one of Zeeman's collaborators, was promoted to the position of 'lector' and head of the spectroscopy section.
Pieter Zeeman's health had been declining since a trip to Rumania in 1934. After his retirement, he lived quietly in Amsterdam, spending the summers in the 'Zonnehof'. Zeeman died on 9 October 1943 at the age of 78 in his house on the Stadhouderskade in Amsterdam and was buried in Haarlem.