References.—“Electricity in the Service of Man,” R. Wormell, London, 1900, p. xiv; Philosophical Transactions, Vol. LI. part i. pp. 171, 340, 366, 373, etc., 389, and Vol. LVII. p. 458; also Hutton’s abridgments, Vol. XI. p. 405; Nollet, “Lettres,” etc., Vol. III. p. 42; “Encycl. Brit.,” article “Electricity”; “Library of Useful Knowledge,” London, 1829, “Electricity,” Nos. 160 and 161.
A.D. 1760.—Mayer (Johann Tobias, Sen.) (1723–1762), one of the most celebrated German astronomers, director of the observatory at Göttingen, is the first to make known the law of the inverse square resulting from actual experimental investigation. This he does in a paper, “Inclination and Declination of the Magnetic Needle, as deduced from theory,” read before the Royal Society at Göttingen, wherein he states that the intensities of the magnetic attractions and repulsions vary inversely as the squares of the distances from the pole of a magnet. Consult “Magnetism,” in the ninth edition of the “Encyclopædia Britannica,” for additional reference to the above paper, also section 14 of the same work for an account of Mayer’s dipping needle as constructed by General Sabine.
References.—Delambre’s notice of the life of J. T. Mayer in the “Biographie Universelle”; Hutton’s “Mathem. Dict.”; Montucla, “Histoire des Mathématiques”; list of his works added to the éloge pronounced by Kaestner, Göttingen, 1762; “Abhandlungen von Galvani und andern,” Prague, 1793; Whewell, “History of the Inductive Sciences,” 1859, Vol. II. pp. 206, 221; Coulomb, “Mémoires Acad. Paris” for 1786 and 1787; “Royal Soc. Cat. of Sc. Papers,” Vol. IV. pp. 311–314; Lambert, “Reports of the Berlin Academy” for 1776.
Mayer (Johann Tobias, Jr.), 1752–1830, is the author of Memoirs on the magnetic needle as well as upon many electrical experiments, of which details may be found in the Journal der Physik of Friedrich A. C. Gren and in the “Comment Soc. Göttingen recent.”
A.D. 1760.—Delaval (E. H.) communicates between 1760 and 1764 several papers to the London Royal Society in reference to experiments made for the purpose of ascertaining the conducting powers of a body in different states. Therein, he shows that animal and vegetable substances lose their conducting powers when reduced to ashes, and that while metals are the best conductors, their oxides are non-conductors. His experiments made with island (Iceland) crystal (well known for its extraordinary property of double refraction), proved that it is affected by heat differently from other substances named, since the temperature necessary to render them electric makes the crystal non-electric. He had a piece of crystal of which, he said, one part became non-electric when greatly heated, while the other part, with the same or even a much greater heat, remained perfectly electric. These experiments did not, however, succeed with Sir Torbern Bergman, who repeated them with great care and who found that island crystal was a conductor in all cases, to whatever degree of heat it was exposed.
References.—Phil. Trans., Vol. LI. part i. p. 83; Vol LII. part i. pp. 353, etc., and part ii. p. 459; also Vol. LIII. part i. pp. 84–98; and Hutton’s abridgments, Vol. XI. pp. 334, 589; Vol. XII. p. 140; Thomas Thomson, “Hist. of Roy. Soc.,” p. 443; Thos. Young, “Course of Lectures,” 1807, Vol. II. p. 679, for notes on Dr. Wm. H. Wollaston’s paper concerning the double refraction of Iceland crystal.
A.D. 1760–1762.—Bergman—Bergmann—(Torbern Olof), celebrated Swedish astronomer, naturalist and chemist, writes several letters to Mr. Wilson, which are read before the Royal Society, Nov. 20, 1760, and March 18, 1762, wherein he alludes to the possibility of electrifying plates of ice in the same manner as plates of glass. In a subsequent letter he details experiments with silk ribbons of different colours, almost as curious as those of which an account has already been given (by Symmer at A.D. 1759), and from which he concludes that there is a certain fixed order regarding positive and negative electricity in which all bodies may be placed while other circumstances remain unchanged.
References.—Bergman’s “Bemerkung ... Isländischen Krystales,” “Comment ... electrica turmalini,” “Elektrische Versuche,” etc., and his other works referred to in the Philosophical Transactions, Vol. LI. p. 907; Vol. LIII. p. 97; Vol. LIV. p. 84; Vol. LVI. p. 236; also Hutton’s abridgments, Vol. XI. pp. 506, 705; Vol. XII. pp. 109, 343; “Nova Acta Soc. Upsal.,” “K. Schwedischen Akad. Abhand.,” “Aus dem Schwed. Magazine,” Phil. Mag., IX. p. 193; “Eng. Cycl.,” Vol. I. pp. 664–665; Gmelin’s “Chemistry,” Vol. I. p. 320; Thomas Thomson, “Hist. of the Royal Society,” London, 1812, pp. 444, 475–477.
A.D. 1761.—The many experiments made at this period by Ebenezer Kinnersley, of Philadelphia, relative to the two contrary electricities of glass and sulphur, are endorsed by his close friend Benjamin Franklin in his Letters at pp. 99, 100 and 102–105. He makes several curious observations on the elongation and fusion of fine iron wires whenever a strong charge is passed through them while in a state of tension, to which Dr. Watson makes special reference in a paper read before the Royal Society. He believes that lightning does not melt metal by a cold fusion, as Dr. Franklin and himself had formerly supposed, and that when it passes, for instance, through the blade of a sword, if the quantity is not very great, it may heat the point so as to melt it, while the broadest and the thickest part may not be sensibly warmer than before.
To ascertain the effects of electricity upon air, Kinnersley devised an instrument which he called an electrical air thermometer, and which is described at p. 626, Vol. VIII of the 1855 “Encyclopædia Britannica.” With this he could show the sudden rarefaction which air undergoes during the passage of the electric spark through it, heat being produced without accompaniment of any chemical change in the heated body.