ZEEMAN,PIETER

(b.Zonnemaire, Zeeland, Netherlands, 25 May 1865; d.Amsterdam, Netherlands, 9 October 1943)

physics.

Zeeman is best remembered for hhis observations in 1896 of the mageto–optic phenomenon that almost immediately was named the Zeeman effect. His experimental discovery was not fortuitous, but the fruition of theoretical views that had motivated attempts over a span of thirty–five years to detect some such interaction between magnetism and light. Zeeman’s initial observations were beautifully comprehended by H.A.Lorentz’ electromagnetic theory, which also served to guide Zeeman in the very early refinement and extension of his discovery. As a result Zeeman and Lorentz shared the 1902 Nobel Prize for physics in recognition of their accomplishment and of the promise, since overshelmingly fulfilled, of the Zeeman effect for contributing to the understanding of spectra and the particulate structure of matter.

Following his elementary education in the small village of Zonnemaire, Zeeman was sent by his parents–Catharinus Farandinus Zeeman, a Lutheran minister, and Wilhelmina Worst–to the secondary school at Zierikzee, five miles away. He subsequently studied classical languages for two years at the gymnsium in Delft in order to satisfy requirements for the university. His early scientific education educatioin seems to have been aequate, foro the published an account of the aurora borealis from Zonnemaire and impressed Kam,erligh Onnes, whom he met at Delft with his grasp of Maxwell’s investigations on heat. Zeeman entered the University of Leiden in 1885 and spent nearly a dozenm yearsthere, working with Kamerlingh Onnes and Lorentz and becoming the latter’s assistant in 1890. For his careful measurements of the Kerr effect, Zeeman won the gold medal of the Netherlands Scientific Society of Haarlem in 1892 and was awarded the doctorate in 1893. After a semester at the Kohlraush Institute in Strasbourg, he returned to the University of Leiden as a Privatdozent In 1895 he marrie Johanna Elisabeth Lebret; they had a son and three daughters.

From January 1897 until his retirement in 1935, Zeeman was associated with the University of Amsterdam. Appointed a lecturer, he was promoted to professor of physics in 1900, succeeded the retiring J. D. van der Waals as director of the Physical Institute in 1908; in 1923 he also became director of the new Laboratorium Physica, later renamed the Zeeman Laboratory. Zeeman the master experimentalist was most effective as a teacher in his regular informal discussions with advanced degree candidates concerning the progress and problems of their laboratory research. He received many awards and honors, including honorary degrees from at least ten universities; was a member of a number of academies; and was associe etranger of the Paris Academy of Sciences. He served as secretary of the Mathematics–Physics Section of the Royal Netherlands Academy of Sciences, Amsterdam, and was a knight of the Order of Orange–Nassau and commander of the Order of the Netherlands Lion.

Zeeman’s was the third magneto–optic effect to be discovered. In 1845 Faraday had observed the first, which related magnetism and propagated light, and served as the experimental basis for subsequent attempts to discover a magnetic influence upon a source of light. The theoretical basis was provided by William Thomson(Lord Kelvin) and by Maxwell’s establishment of the electromagnetic nature of light. Zeeman interrupted his measurements of the Kerr effect(the second magneto–optic effect to be discovered) to improvise an experiment, which proved unsuccessful, seeking some change in the spectrum of a sodium flame that was burning in a magnetic field. About a year later Zeeman learned that Faraday had attempted this experiment without success in 1862. His reaction was that if Faraday, whom he considered the greatest experimental genius of all time, had thought the experiment worth doing, then he could well afford to repeat it again, using the best spectroscopic apparatus and a specially designed electromagnet. Thus equipped, Zeeman observed that the D–lines of the sodium spectrum were decidedly broadened. Within a few weeks he obtained this broadening for other spectral lines and for the related absorption spectra, and convincingly demonstrated that the broadening was a direct effect of the magnetic field.

These results were presented to the Amsterdam Academy of Sciences on 31 October 1896. At the next meeting of the Academy, four weeks later, Zeeman reported that the Lorentz theory not only comprehended his initial findings but also had been used to predict that the light from the edges of the magnetically broadened lines should be polarized–and, further, that he had observed this polarization. The Lorentz theory also provided the equation

where T is the natural period of vibration of an ion of charge e and mass m and T′ is its period in a magnetic field of strength H. This equation enabledl Zee,amn. from his measurements of H.T,and T′, to calculate the charge–to–mass ratio of the vibrating “ion” of the sodium atom. His next paper established that the “ion” was negatively charged.

In the spring of 1897, after his move to the University of Amsterdam, Zeeman resolved a magnetically “broadened” spectral line into the triplet of distinct polarized components that the Lorentz theory predicted for a sufficiently intense magnetic field. This in a very real sense was the peak of the Zeeman–Lorentz investigation of the Zeeman effect. For his more exacting measurements at this time, Zeeman had to travel to the University of Groningen to use Hermann Haga’s superior spectroscopic apparatus; and by the end of 1897, the limitations of his research facilities at Amsterdam proved decisive for Zeeman. The main deficiency, which persisted until the building of his own laboratory, was in the mountings of his spectroscope. The most interesting and demanding measurements required an isolated and rigid mounting system to ensure sharp definition in the photographs. Zeeman’s attempts in this regard were usually spoiled (less than one photograph in thirty was usable) by vibrations due either to human movement on the same floor as his laboratory or to the traffic of Amsterdam–even in the middle of the night. After a promising but qualitative anticipation of a fundamental relationship between Zeeman effect patterns and the laws of spectral series (subsequently called Preston’s law), Zeeman felt compelled to abandon this suggestive investigation and turn to less exacting studies of the Zeeman effect and related matters. For these researches, which fully engaged him over the next fifteen years, the magneto–optic theory of Woldemar Voigt performed the same roles that Lorentz’ theory had for Zeeman’s earlier investigations.

During World War I, Zeeman initiated a systematic redetermination of the velocity of propagation of light in moving transparent media. Early in the nineteenth century Fresnel’s optical theory had required that light propagated longitudinally with respect to moving glass would suffer a velocity chage of where the factor was the Fresnel coefficient μ was the index of refraction, and v was the velocity of the glass. In the middle of the century Fizeau had used interference techniques to obtain experimental support for the Fresnel coefficient in the case of light traversing flowing water. Thirty–five years later Michelson and Morley repeated Fizeau’s experiment, and with their more precise interferometer they obtained a value in closer agreement with the Fresnel coefficient. Zeeman’s interest in this question was generated by the theoretical investigations that Lorentz conducted in 1895 and subsequently reconsidered in terms of relativity theory. By taking into account the dispersion of light in the medium, Lorentz deduced that the coefficient must be where is λ the wavelength of light. In two papers of 1915 and 1916, Zeeman essentially repeated the Michelson–Morley experiment with water and showed that the experimental value of the coefficient did vary with the wavelength of the light used, and that within his limits of experimental error it confirmed the Lorentz rather than the Fresnel expression. In 1919 and 1920 Zeeman collaborated with others to communicate three additional papers dealing with the same measurements, but for quartz and glass rather than water. The use of rapidly moving solid substances imposed extraordinary experimental difficulties that Zeeman painstakingly surmounted in order to obtain experimental results further supporting the Lorentz refinement of the Fresnel coefficient.

In 1918 Zeeman published the results of another extremely meticulous set of experiments that also carried profound implificatioins for relativity theory. These measurements, which Zeeman conducted at his country home after determining that they could not be made in the laboratory because of the ever present vibrations, established an equality of the inertial and gravitational mass for certain crystals and radioiactive substances to within one part in twenty or thirty million.

With the facilities of his new laboratory available after 1923, Zeeman, in collaboration with others, finally turned to experiments involving precision measurements of the Zeeman effect. Most notable were a series of investigations of the magnetic resolution of the spectral lines of certain noble gases and, in 1932, a detailed and beautifully presented study of the hyperfine structure and Zeeman effect of the strong spectral lines of rhenium, both of which confirmed the value of the nuclear moment of the two rhenium isotopes.

BIBLIOGRAPHY

I. Original Works. Zeeman’s main magneto–optic papers, written in the period 1896-1913, were collected and republished in the commemorative volume Verhan delingen van Dr. P.Zeeman over magneto–optische Verschijnselen, H. A. Lorentz, H. Kamerlingh Onnes, I. M. Graftdijk, J. J. Hallo, and H. R. Woltjer, eds. (Leiden, 1921). In Researches in Magneto–Optics (London, 1913) Zeeman discussed his own and others’ contributions during this same period to Zeeman effect and closely related phenomena, and appended a very valuable bibliography. Nearly all of Zeeman’s published papers are cataloged in Poggendorff, IV, 1682; V, 1404–1405; and VI, 2957–2958.

II. SECONDARY LITERATURE. For information on Zeeman see the “Biography” in Nobel Lectures. Physics, 1901–1921 (Amsterdam, 1967), 41–44; Lord Rayleigh, “Pieter Zeeman 1865–1943,” in Obituary Notices of Fellows of the Royal Society of London, 4 (1944), 591–595; and H. Kamerlingh Onnes. “Zeeman’s Ontdekking van het naar hem genoemde Effect,” in Physica1 (1921), 241–250.

James Brookes Spencer