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XL VI. Magnetic Figures illustrating Electrodynamic Relations. 

 Bit Silvanus P.' Thompson, D.Sc, B.A., F.R.A.S., Pro- 

 fessor of Experimental Physics inUniversity College, Bristol*. 

 [Plates VI. and VII.] 



IN a preliminary communication to the Physical Society in 

 February of the present year, the author announced a 

 method of studying and illustrating the known laws of the 

 mutual attractions or repulsions of conductors traversed by 

 electric currents. The present paper is a complete statement 

 of the facts obtained in the experimental research which formed 

 the basis of that communication. 



While preparing a set of magnetic currents to illustrate the 

 mutual actions of magnet-poles, it occurred to the writer that 

 the mutual attractions and repulsion of currents might be il- 

 lustrated in a similar manner by the figures formed with iron 

 filings. He was awaref at that time that the lines of force of 

 a straight conductor carrying a current were a series of con- 

 centric circles lying in a plane to which the conductor was 

 normal. The series of figures now published originates, there- 

 fore, with the discovery of Faraday that the seat of the mutual 

 actions of currents and of magnets must be sought in the sur- 

 rounding medium. Since the communication of the prelimi- 

 nary notice, the writer has learned that one or two of the figures 

 had been previously and independently observed by Professor 

 F. Guthrie, but not published. Two others, ISfos. 4 and 5 of 

 the present series, are imperfectly given by Faraday in figures 

 18 and 19 of plate iii. in the third volume of his ' Experimental 

 Researches ' (Series Twenty-ninth) ±, and without reference to 

 the conclusions to be derived from their forms, which Faraday 

 apparently overlooked § . 



The method employed for preserving the figures has been 

 uniform throughout the series. Plates of glass, 3i inches long 

 by 3J inches broad, were coated with a solution of gum-arabic 

 and gelatine, and were then carefully dried. When the ar- 



* Communicated by the Physical Society. 



t See Faraday, ' Experimental Researches in Electricity/ vol. iii. p. 

 400, § 3239, and plate iii. fig. 17 ; Guthrie, ' Magnetism and Electricity,' 

 p. 254, fig. 225 ; Clerk-Maxwell, ' Electricity and Magnetism/ vol. ii. 

 art. 477. 



% And ' Phil. Trans. 1852, p. 137. 



§ The attention of the -writer has also been drawn to a statement in the 

 American Journal of Science for 1872, p. 263, by Professor A. M. Mayer, 

 that he has obtained magnetic " spectra " from electric currents in a manner 

 somewhat similar to that now described. The figures have, however, re- 

 mained unpublished and undescribed, so that the writer has no means of 

 learning how far the substance of the present communication may have 

 been anticipated. 



