Cantor lectures on the electromagnet / by Silvanus P. Thompson ; delivered before the Society of Arts, January 20, 27, February 3, 10, 1890.

  • Thompson, Silvanus P. (Silvanus Phillips), 1851-1916.
Date:
1890
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Licence: Public Domain Mark

Credit: Cantor lectures on the electromagnet / by Silvanus P. Thompson ; delivered before the Society of Arts, January 20, 27, February 3, 10, 1890. Source: Wellcome Collection.

Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.

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    he wants a special form having the most rapid action attainable ; in yet another he must sacrifice everything else to attain maximum action with minimum weight. Toward the solution of such practical problems as these the old theory of magnetism offered not the slightest aid. Its array of mathematical symbols was a rnockery. It was as though an engineer asking for rules to enable him to design the cylinder and piston of an engine were confronted with receipts how to estimate the cost of painting it. Gradually, however, new light dawned. It became customary, in spite of the mathe- maticians, to regard the magnetism of a magnet as something that traverses or circu- lates around a definite path, flowing more freely through such substances as iron, than through other relatively non-magnetic materials. Analogies between the flow of electricity in an electrically-conducting cir- cuit, and the passage of magnetic lines of force through circuits possessing magnetic conductivity, forced themselves upon the minds of experimenters, and compelled a mode of thought quite other than the pre- viously accepted. So far back as 1821, Gumming* experimented on magnetic con- ductivity. The idea of a magnetic circuit was more or less familiar to Ritchie,f Sturgeon,| Dove,§ Dub,II and De La Rive,^ the last- named of whom explicitly uses the phrase, a closed magnetic circuit. Joule** found the maximum power of an electromagnet to be proportional to the least sectional area of the entire magnetic circuit, and he con- sidered the resistance to induction as propor- tional to the length of the magnetic circuit. Indeed, there are to be found scattered in Joule's writings on the subject of magnetism, some five or six sentences, which, if collected together, constitute a very full statement of the whole matter. Faraday,f-j- considered that he had proved that each magnetic line of force constitutes a closed curve; that the path of * Camb. Phil. Trans., April 2, 1821. t ** Phil. Mag , series iii., vol. iii., p. 122. t Ann. of Electr., xii., p. 217. ? Pogg. Ann., xxix., p. 462, 1833. See also Pogg. Ann., xliii., p. 517, 1838. II Dub. Elektromagnetismus (ed. 1861), p. 401; and Pogg. Ann., xc, p. 440, 1853. II De La Rive. Treatise on Electricity (Walker's translation), vol. i, p. 292, ** Ann. of Electr.. iv., 59, 1839; v., 195, 1841 ; and Scientific Papers, pp. 8, 34, 35, 36. tt Experimental Researches, vol. iii,, art. 3117, 3228, 3230, 3260, 3271, 3276, 3294, and 3361. netic conductivity of the masses disposed in proximity ; that the lines of magnetic force were strictly analogous to the lines of electric flow in an electric circuit. He spoke of a magnet surrounded by air being like unto a voltaic battery immersed in water or other electrolyte. He even saw the existence of a power, analogous to that of electromotive- force in electric circuits, though the name, magneto-motive force, is of more recent origin. The notion of magnetic conductivity is to be found in Maxwell's great treatise (vol. ii., p. 51), but is only briefly mentioned. Rowland,* in 1873, expressly adopted the reasoning and language of Faraday's method in the working out of some new results on magnetic permeability, and pointed out that the flow of magnetic lines of force through a bar could be subjected to exact calcula- tion; the elementary law, he says, is similar to the law of Ohm. According to Rowland, the magnetizing force of the helix was to be divided by the resistance to the lines of force; a calculation for magnetic circuits which every electrician will recognise as being precisely like Ohm's law for electric circuits. He applied the calculations to determine the permeability of certain speci- mens of iron, steel, and nickel. In 1882,-]- and again in 1883, Mr R. H. M. BosanquetJ brought out at greater length a similar argument, em- ploying the extremely apt term Magneto- motive Force, to connote the force tending to drive the magnetic lines of induction through the magnetic resistance or, as it will be frequently called in these Lectures, the magnetic reluctance of the circuit. In these papers the calculations are reduced to a system, and deal not only with the specific properties of iron, but with problems arising out of the shape of the iron. Bosanquet shows how to calculate the several resistances (or reluctances) of the separate parts of the circuit, and then add them together to obtain the total resistance (or reluctance) of the magnetic circuit. Prior to this, however, the principle of the magnetic circuit had been seized upon by Lord Elphinstone and Mr. Vincent, who pro- * Phil. Mag., series iv., vol. xlvi., August 1873, On Magnetic Permeability and the Maximum of Magnetism of Iron, Steel, and Nickel. t Proc. Ro3'al Soc, xxxiv., p. 445, December, 1882. t Phil. Mag., series v., vol. xv., p. 205, March, 1883. On Magneto-Motive Force. Also id., vol. xix., February, 1885, and Proc. Roy. Soc, No. 223, 1883. See also Electrician, xiv., p. 291, February 14th, 1885.