Clay (Claij), Jacob

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Clay (Claij), Jacob

(b. Berkhout, Netherlands, 18 January 1882; d. Bilthoven, Netherlands, 31 May 1955),


Clay studied physics at Leiden University and was assistant to Kamerlingh Onnes from 1903 to 1907. He received his doctorate in 1908 with the thesis “De galvanische weerstand van metalen en legeeringen bij lage temperaturen” (“The Galvanic Resistance of Metals and Alloys at Low Temperatures”). He taught in a secondary school at Leiden in 1906 and at Delft from 1907 to 1920. At the Delft Technological University he was privaat-docent in natural philosophy from 1913. In 1920 he became professor of physics at the Bandung Technological University (now in Indonesia) and in 1929 took the same post at the University of Amsterdam.

With interest in general physics and its teaching Clay combined a predilection for philosophy, starting from Hegel, and on the experimental side, for atmospheric electricity. In Bandung, assisted by his physicist wife and his children, he investigated the then rather new subject of cosmic radiation. On voyages from Indonesia to the Netherlands he discovered the latitude effect, a diminution in the intensity of cosmic radiation in the equatorial regions that is caused by the earth’s magnetic field, thus establishing the presence of charged particles in primary cosmic radiation. Against doubts of other investigators he firmly established the latitude effect and, with the aid of pupils, made further investigations after moving to Amsterdam University. In this connection he worked for the improvement of electric measurements of ionization in general.

Clay’s straightforward nature and honest diplomacy made him a good executive as director of scientific institutions. Most of his scientific work is published in Physica (The Hague).


I. Original Works. Among Clay’s books are Rayons cosmiques (Paris, 1938), written with P. M. S. Blackett and G. Lemaître; De ontwikkeling van het denken (Utrecht, 1950); Atmosferische electriciteit (The Hague, 1951); and Wetenschap en maatschappij (Amsterdam, 1952), as well as books on philosophy and measurement of radioactivity.

II. Secondary Literature. Clay is discussed in Gedenkboek Athenaeum-Universiteit Amsterdam (1932); Nederlands tijdschrift voor natuurkunde, 18 (1952), 241, and 21 (1955), 149; and Jaarboek van de K. Nederlandse Akademie van wetenschappen, gevestigd te Amsterdam (1955–1956), p.209.

J. A. Prins