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Le Roy, Jean-Baptiste

Le Roy, Jean-Baptiste

(b. Paris, France, 15 August 1720; d. Paris, 21 January 1800),

physics, scientific instrumentation

Son of the renowned clockmaker Julien Le Roy, Jean-Baptiste Le Roy was one of four brothers to achieve scientific prominence in Enlightenment France; the others were Charles Le Roy (medicine and chemistry), Julien-David Le Roy (architecture), and Pierre Le Roy(chronometry). Elected to the Académie Royale des Sciences in 1751 as adjoint géomètre, Le Roy played an active role in technical as well as administrative aspects of French science for the next half-century. He was elected pensionnaire mécanicies in 1770 and director of the Academy for 1773 and 1778, and became both a fellow of the Royal Society and a member of the American Philosophical Society in 1773.

Le Roy’s major field of enquiry was electricity, a subject on which European opinion was much divided at mid-century. The most prominent controversy engaged the proponents of the Abbé Nollet’s doctrine of two distinct streams of electric fluids (outflowing and inflowing) and the partisans of Benjamin Franklin’s concept of a single electric fluid. This debate intensified in France in 1753 with an attack on Franklin’s views by Nollet. Le Roy, later a friend and correspondent of Franklin, defended his single-fluid theory and offered considerable experimental evidence in support thereof. He played an important role in the dissemination of Franklin’s ideas, stressing particularly their practical applications, and published many memoirs on electrical machines and theory in the annual Histoires and Mémoires of the Academy and in the Journal de Physique.

A regular contributor to the Encyclopédie, Le Roy wrote articles dealing with scientific instruments. The most important of these included comprehensive treatments of “Horlogerie,” “Télescope,” and “Électrométre” (in which Le Roy claimed priority for the invention of the electrometer). He also promoted the use of lightning rods in France, urged that the Academy support technical education, and was active in hospital and prison reform. After the Revolutionary suppression of royal academies, Le Roy was appointed to the first class of the Institut National (section de mécanique) at its formation in 1795.


I. Original Works. Le Roy wrote no extended treatise on electricity. His more important publications include “Mémoire sur l’électricité, où l’on montre par une suite d’expériences qu’il y a deux espèces d’électricités, l’une produite par la condensation du fluide électrique,& l’autre par sa raréfaction,” in Mémoires de l’ Académie royale des sciences (1753),447-474; “Mémoire sur un phénoméne électrique intéressant et qui n’avoit pas encore été observé; ou sur la différence des distances auxquelles partent les étincelles entre deux corps métalliques de figures différentes, selon que l’un de ces deux corps est électrifié, & que l’autre lui est présenté,”ibid. (1766).541-546:“Mémoire sur les verges ou barres métalliques, destinées à garantir les édifices des effets de la foudre,” ibid. (1770),53-67; “Mémoire sur une machine à électriser d’une espéce nouvelle,”ibid., pt.1 (1772), 499-512; and“Mémoire sur la forme des barres ou des conducteurs métalliques destinées à preserver les édifices des effets de la foudre, en transmettant son feu à la terre,” ibid. (1773), 671-686.

For Le Roy’s ideas on hospitals, see Précis d’un ouvrage sur les hôpitaux, dans lequel on expose les principes résultant des observations de physique et de médecine qu’on doit avoir en vue dans la construction de ces édifices, avec un projet d’hôpital disposé d’ aprés ces principes(Paris, n.d.).

II. Secondary Literature. There is no biography of Le Roy. Details concerning his life and work can be found in Louis Lefévre-Gineau, Funérailles du citoyen le Roy (Paris, 1801); and F. Hoefer, ed., Nouvelle biographie generale, XXX (1859), 891. Franklin’s letters to Le Roy are in Albert Henry Smyth, ed., The Writings of Benjamin Franklin, 10 vols. (New York, 1905-1907),passim.

Martin Fichman

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