SCIENCE. 



[Vol. XVI. No. 40? 



Tiously accepted. So far back as 1821, Cumtning^ experi- 

 mented on magnetic conductivity. The idea of a magnetic 

 circuit was more or less familiar to Ritchie," Sturgeon," Dove,'' 

 Dub,'* and De la Rive,** the last named of whom explicitly 

 uses the phrase "a closed magnetic circuit." Joule' found 

 the maximum power of an electro-magnet to be proportional 

 to "the least sectional area of the entire magnetic circuit," 

 and he considered the resistance to induction as proportional 

 to the length of the magnetic circuit. Indeed, there are to 

 be found scattered in Joule's writings on the subject of mag- 

 netism some five or six sentences, which, if collected to- 

 gether, constitute a very full statement of the whole matter. 

 Faraday* considered that he had proved that each demag- 

 netic line of force constitutes a closed curve; that the path 

 of these closed curves depended on the magnetic conductivity 

 of the masses disposed in proximity ; that the lines of mag- 

 netic force were strictly analogous to the lines of electric 

 flow in an electric circuit. He spoke of a magnet sur- 

 rounded 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 electro-motive 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 calculation. The elemen- 

 tary law, he says, "is similar to the law of Ohm.'' Accord- 

 ing to Rowland, the "magnetizmg force of helix" was to be 

 divided by the "resistance to the lines of force,'' — a calcula- 

 tion for magnetic circuits which every electrician will rec- 

 ognize as precisely as Ohm's law for electric circuits. He 

 applied the calculations to determine the permeability of cer- 

 tain specimens of iron, steel, and nickel. In 1882," and 

 again in 1883, Mr. R. H. M. Bosanquet" brought out at greater 

 length a similar argument, employing the extremely apt 

 term "magneto-motive force" to connote the force tending 

 to drive the magnetic lines of induction through the "mag- 

 netic 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 speciflc 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 sepa- 

 rate parts of the circuit, and then add them together to ob- 

 tain the total resistance (or reluctance) of the magnetic cir- 

 cuit. 



' Cambridge Philosophical Transactions, April 2, 18S1. 



2 Philosophical Magazine, series ill. vol. ili. p. 122. 



2 Anuals of Electricity, sil. p. 217. 



* Poggendorf's Annalen, 183.3, xxlx. p. 462; 1838, xliil. p. 51". 



^ Dub's Elektromagnetismus (ed. 1861), p. 401; Poggendorf's Annalen, 1853, 

 xc. p. 440. 



" De la Rive's Treatise on Electricity (Walker's translation), 1. p. 292. 



' Annals of Electricity, 1839, Ir. p. 59; Ibid, 1841, v. p. 195; Scientific Papers, 

 pp. 8, 34, 35, 36 



8 Experimental Eesearches, ili. arts. 3117, .32i8, 3'230, 3260, 3271, 3376, 

 3294, and 3361. 



^ Philosophical Magazine, series iv. vol. xlvl, August, 1873, "On Magnetic 

 Permeability and the Maximum of Magnetism of Iron, Steel, and NickeL" 



'» Proceedings of the Royal Society, xxxlv. p. 445, December, 1882 



^^ Philosophical Magazine, series v. vol. xv. p. 205, March, 1883, *0n Mag- 

 neto-Motive Force;" Ibid, xis. Eebruary, 1885; Proceedings of the Royal Soci- 

 clety, 1883, No. 223; Electrician, xiv. p. 291, Feb. 14, 1885. 



Prior to this, however, the principle of the magnetic cir- 

 cuit had been seized upon by Lord Elphinstone and Mr. 

 Vincent, who proposed to apply it in the construction of the- 

 dynamo-electric machines. On two occasions' they commu- 

 nicated to the Royal Society the results of experiments to 

 show that the same exciting current would evoke a larger- 

 amount of magnetism in a given iron structure if that iroit 

 structure formed a closed magnetic circuit than if it were- 

 otherwise disposed. 



In recent years the notion of the magnetic circuit has beeo 

 vigorously taken up by the designers of dynamo-machines, 

 who indeed base the calculation of their designs upon this 

 all-importaut principle. Having this, they need no laws of 

 inverse squares of distances, no magnetic moments, none of 

 the elaborate expressions for surface distribution of magnet- 

 ism, none of the ancient paraphernalia of the last century. 

 The simple law of the magnetic circuit and a knowledge of 

 the properties of iron are practically all they need. About 

 four years ago, much was done by Mr. Gisbert Kapp^ and 

 by Drs. J. and E. Hopkinson^ in the application of these con- 

 siderations to the design of dynamo-machines, which previ- 

 ously had been a matter of empirical practice. To this end 

 the formula of Professor Forbes* for calculating magnetic 

 leakage, and the researches of Professors Ayrton and Perry^ 

 on magnetic shunts, contributed a not unimportant share. 

 As the result of the advances made at that time, the subject 

 of dynamo design was reduced to an exact science. 



It is the aim and object of the present course of lectures 

 to show how the same considerations which have been applied 

 withsuchgreatsuccessto the subject of the design of dynamo- 

 electric machines may be applied to the study of the electro- 

 magnet. The theory and practice of the design and con- 

 struction of electro-magnets will thus be placed, once for 

 all, upon a rational basis. Definite rules will be laid down 

 for the guidance of the constructor, directing him as to the 

 proper dimensions and form of iron to be chosen, and as to 

 the proper size and amount of copper wire to be wound upon 

 it in order to produce any desired result. 



First, however, an historical account of the invention will 

 be given, followed by a number of general considerations- 

 respecting the uses and forms of electro-magnets. These 

 will be followed by a discussion of the magnetic properties 

 of iron and steel and other materials, some account being- 

 added of the methods used for determining the magnetic 

 permeability of various brands of iron at different degrees of 

 saturation. Tabular information is given as to the results 

 found by different observers. In connection with the mag- 

 netic properties of iron, the phenomenon of magnetic hys- 

 teresis is also described and discussed. The principle of the 

 magnetic circuit is then discussed with numerical examples, 

 and a number of experimental data respecting the perform- 

 ance of electro-magnets are adduced, in particular those 

 bearing upon the tractive power of electro-magnets. The 

 law of traction between an electro-magnet and its armature 



1 Proceedings of the Royal Society, 1879, xxix. p. 292; Ibid, 1880, xxx. p. 

 287; Electrical Revie-w, 1880, vili. p. 134. 



2 The Electrician, 1886-86, xiv. xv. xvl.; Proceedings of the Institute of Civil 

 Engineers, 1885-86, Ixxxlli.; Joumal of the Society of Telegraphic Engineers, 

 1886, XV. p. 524. 



3 Philosopbical Transactions, 1886, part i. p. 331; The Electrician, 1886, xvlli. 

 pp. 39, 63, 86. 



« Journal of the Society of Telegraphic Engineers, 1886, xv. p. 555. 

 ' Ibid, p. 530. 



