References.—Alexander Jones, “Sketch of the Elect. Teleg.,” New York, 1852, p. 7; Charles F. Briggs, “Story of the Telegraph,” 1858, p. 18.
A.D. 1729.—Hamilton (James), who became sixth Earl of Abercorn—also called Lord Paisley—publishes “Calculations and Tables relating to the attractive virtue of loadstones ...” containing very valuable data and wherein he is the first to give the true law of the lifting capacity of magnets, as follows: “The principle upon which these tables are formed is this: That if two loadstones are perfectly homogeneous, that is if their Matter be of the same specifick parity, and of the same virtue in all parts of one stone, as in the other; and that like parts of their surfaces are cap’d or arm’d with iron; then the weights they sustain will be as the squares of the cube roots of the weights of the loadstones; that is, as their surfaces.”
Gilbert treats of armed loadstones, Book II. chaps. xvii-xxii. In connection with the increased energy which magnets acquire by being armed, that is, fitted with a cap of polished iron at each pole, Dr. Whewell remarks that it is only at a later period any notice was taken “of the distinction which exists between the magnetical properties of soft iron and of hard steel; the latter being susceptible of being formed into artificial magnets, with permanent poles; while soft iron is only passively magnetic, receiving a temporary polarity from the action of a magnet near it, but losing this property when the magnet is removed. About the middle of the last century various methods were devised of making artificial magnets, which exceeded in power all magnetic bodies previously known” (“Hist. of the Ind. Sc.,” 1859, Vol. II. p. 220).
Hamilton alludes to a loadstone weighing 139 grains, with a lifting power of 23,760 grains! We have referred, amongst others, to the loadstone belonging to Sir Isaac Newton at A.D. 1675, and to the wonderful collection belonging to Mr. Butterfield at A.D. 1809. A loadstone weighing twelve ounces, capable of lifting sixty pounds of iron, is referred to in Terzagus, “Musæum Septalianum,” 1664, p. 42, while another weighing two and a half grains and lifting 783 grains is mentioned at p. 272, Vol. III. of the “Records of General Science”; and Salviatus (“Dialogues of Galileo,” Dial. III) alludes to one in the Academy of Florence which, unarmed, weighed six ounces and could lift but two ounces, but when armed had a lifting power of 160 ounces. At pp. 317–318, Part III of Nehemiah Grew’s “Musæum Regalis Societatis,” London, 1681—also 1686—allusion is made to a loadstone found in Devonshire, weighing about sixty pounds, which moved a needle nine feet distant. Grew then refers to Athan. Kircher and to Vincent Leotaud as having published what is said of the loadstone by Gilbert and others, and he likewise states: “Those that travail through the vast deserts of Arabia, have also a needle and a compass whereby they direct themselves in their way, as Mariners at sea [Majoli, ‘Colloquia’]; the power of the magnet dependeth not upon its bulk—the smaller being usually the stronger....”
References.—Phil. Trans. for, 1729–1730, No. 412, Vol. XXXVI. p. 245, and for July 1888, also Hutton’s abridgments, Vol. VII. p. 383; V. T. M. Van der Willigen, “Arch. du Musée Teyler,” 1878, Vol. IV; Jacobi Rohaulti, “Physica,” 1718, Part III. cap. 8, p. 403, or the English translation by Dr. Clarke, 1728, Vol. II. p. 181; P. W. Hacker, “Zur theorie des magnetismus,” Nürnberg, 1856; Ath. Kircher, “Magnes. ...” 1643, lib. i. part ii. p. 63; Daniel Bernoulli, “Acta Helvetica,” 1758, Vol. III. p. 223; Nic. Cabæus “Philosophia Magnetica,” 1629, lib. iv. cap. 42, p. 407; Kenelme Digby, “The Nature of Bodies,” 1645, Chap. XXII. p. 243; “Dict. of Nat. Biog.” Vol. XXIV. p. 185.
A.D. 1729–1730.—Savery (Servington), English mechanician, succeeds in imparting magnetism to hard steel bars three-fourths of an inch square and sixteen inches long, by fitting one bar with an armature at each end and touching other bars with it whilst held in the magnetic meridian in the line of the inclined needle.
It was shown by Savery that his artificial magnets were preferable to loadstones. The first recorded attempt to make artificial magnets is credited to one John Sellers, believed to be the author of “The Practical Navigator,” of which the earliest edition appeared in 1669, and of “The Coasting Pilot,” published about 1680. An “Answer to Some Magnetical Inquiries Proposed in (the preceding) No. 23, pp. 423–424,” will be found in Phil. Trans. for 1667, Vol. II. pp. 478–479 and in the following abridgments: Baddam, 1745, Vol. I. p. 86; Hutton, Vol. I. p. 166 (as of No. 26, p. 478); John Lowthorp, Vol. II. p. 601. Reference is likewise made to this invention of Sellers at Vol. I. p. 86 of the “Memoirs of the Royal Society,” London, 1739, and in a paper by Réaumur, in the “Mémoires de l’Académie Française” for the year 1723.
References.—Savery, “Magnetical Observations and Experiments,” also Phil. Trans., Vol. XXXVI. pp. 295–340; and the following abridgments: Hutton, Vol. VII. p. 400; Reid and Gray, Vol. VI. p. 166; Eames and Martyn, Vol. VI. p. 260; Baddam, 1745, Vol. IX. p. 57; Geo. Adams, “Essay on Electricity,” 1785, p. 451.
A.D. 1731.—On the 25th of November the Royal Society were honoured by a visit from the Prince of Wales and the Duke of Lorraine, the last named being enrolled as a member during the evening. Experiments were performed “On the strength of Lord Paisley’s loadstone,” “On Dr. Frobenius’s phlogiston,” and “On the electrical observations of Mr. Stephen Grey.” These experiments which, it is said, “succeeded notwithstanding the largeness of the company,” showed the facility with which electricity passes through great lengths of conductors and are worth noting as being the first of their nature.
A.D. 1732.—Régnault (Le Père Noël) gives in “Les Entretiens Physiques,” etc., Vol. I. Nos. 15 and 16, the tables of the declination at Paris from the years 1600–1730, and treats at length of the merits of the loadstone and of the magnetic needle.