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You are citing an entry from an edited book. The author of the entry is P.G. Bergmann and the entry is titled "Relativity" from The New Encyclopedia Britannia, edited by John Wright and Alice James. This encyclopedia was published in 2003, and the article is on pages 501-508. The encyclopedia was published in Chicago, IL by Encyclopedia Britannica.
What is Bergmann, P. G. (1993). Relativity. In J. Wright, & A. James (Eds.), The New Encyclopedia Britannica (pp. 501-508). Chicago, IL: Encyclopedia Britannica.