focaloidn.
/ˈfəʊkəlɔɪd/, U.S. /ˈfoʊkəˌlɔɪd/ Mathematics.
A shell (shell n. 12b) bounded by two confocal ellipsoidal surfaces, esp. (in early use) one that is infinitely thin.
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focaloid1879
homoeoid1883
1879 W. Thomson & P. G. Tait Treat. Nat. Philos. (new ed.) I: Pt. i. §494 The attraction of a homogeneous solid ellipsoid is the same through all external space as the attraction of a homogeneous focaloid of equal mass coinciding with its surface.
1901 Philos. Trans. (Royal Soc.) A. 197 508 It is well known that the potential of a solid ellipsoid externally is equal to that of a ‘focaloid’ shell of the same mass coincident with its external surface.
1921 J. Edwards Treat. Integral Calculus I. xxii. 857 Find the mass of a thick focaloid..and the density of each point.
2014 Monthly Notices Royal Astron. Soc. 440 1520/1 Section 5 is devoted to the special case of compressible focaloids, where the shells are bounded by confocal ellipsoids.
This entry has been updated (OED Third Edition, September 2016; most recently modified version published online March 2022).
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n.1879