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XXIX. On the Influence of a Plane of Transverse Section on the 

 Magnetic Permeability of an Iron Bar. By Professor J. A. 

 Ewjng, F.R.S., University College, Dundee, and Mr. 

 William Low*. 



IT has been remarked by Professor J. J. Thomson and Mr. 

 H. F. Newall (Proe/ Camb. Phil. Soc. Feb. 1887) that 

 when an iron bar is cut across, and the ends are brought into 

 contact, the magnetic permeability is notably reduced. 



Our attention was drawn to the matter in experimenting 

 on the magnetization of iron in very strong fields, by what 

 we have called the " isthmus" method |, in which a bobbin with 

 a narrow neck was magnetized between the poles of a large 

 electromagnet. We found that when a solid bobbin was 

 used the central neck was much more strongly magnetized 

 than when the neck was a distinct piece separated by planes 

 of transverse section from the conical ends ; and we were 

 forced to ascribe the defect of permeability in the latter case 

 to the existence of two planes of section at the ends of the 

 short central piece of metal in which the induction was being 

 measured. To examine the influence of transverse section 

 more fully, and to see how far that influence is modified when 

 the cut pieces are pressed against one another, we made a 

 series of experiments in which the permeability of cut and 

 uncut bars was determined by a method similar to that used 

 by Dr. J. HopkinsonJ. The bar to be tested was let into a 

 massive yoke of wrought iron, which provided a double path 

 for the return of the lines of induction from end to end. The 

 effect of this is to get rid in great measure of the self- 

 demagnetizing influence of the bar's ends, so that the metal 

 may be tested in a condition approximating to endlessness. 

 Fig. 1 shows the apparatus : the bar a is sunk in holes in the 

 yoke bb. The lower end of the bar abuts against a set screw 

 c, and on the upper end there is an iron plunger d, through 

 which a stress of compression can be produced in the bar by 

 loading the lever e. The diameter of the bar was 0*79 centim. 

 (section 0'49 square centim.) ; the clear length of the bar 

 between the inside faces of the yoke was 12*7 centims., and 

 the cross section of the yoke gave an iron surface more than 

 100 times greater than the cross section of the bar. The 



* Eead in abstract before Section A of the British Association at Man- 

 chester, and now published with later additions. Communicated by 

 Professor Ewing. 



t Proc. Roy. Soc. vol. xlii. p. 200. 



\ " Magnetization of Iron," Phil. Trans. 1885, Part II. p. 455. 



