Pole in the Electrostatic System of Units. 361 



tion between the medium inside the region of the solenoid 

 corresponding to the substance of the magnetic shell, and that 

 outside. He over and over again lays stress upon the fact that 

 artificial solenoids can only be compared with magnetic shells 

 for the space outside the shells, and that the line of integration 

 must never be allowed to thread the circuits. Let us follow this 

 out and see what it means when applied to the above question. 



I will assume it possible (for it certainly is theoretically 

 possible) to imitate any steel magnet whatever by a proper 

 arrangement of electric circuits, both being at present im- 

 mersed in a non-magnetic (i. e. non-magnetizable or /c*=l) 

 medium. The two arrangements are completely equivalent 

 for all the region outside both — the region outside both being 

 defined by the shape of the steel. For the comparison is not to 

 be urged within the steel, because of the magnetized surfaces, 

 which would have to be cut through, a circumstance which 

 would completely alter all the conditions ; and it is not to be 

 urged within the region of space near the solenoid which is the 

 counterpart of the steel-occupied region, simply because here 

 there are no magnetized surfaces to be cut through, and there- 

 to the conditions will be continuous. 



Now take some non-magnetic medium, which for shortness 

 I will call " clay," mould it into the shape of the steel, and 

 place it in or around the solenoid so as to mechanically define 

 the limits of the "outer region." And now immerse both 

 magnet and solidified solenoid in any medium for which //. 

 differs from unity: I venture to assert that the equivalence 

 which existed in air will be entirely maintained in the mag- 

 netic medium — even though that medium be iron or bismuth, 

 — and that, for both, the magnetic intensity at any point will 

 be its air-value divided by fi. 



Still keeping both the things in the magnetic medium, 

 remove the clay from the solenoid and permit the medium to. 

 flow into the space it occupied. If what I said before is true, 

 the solenoid will now be too strong for the magnet, for the mag- 

 netic permeability of the interior will increase its effect yu, times, 



while that of the exterior will, as before, diminish it -th : so 



that the effect of the solenoid completely immersed in the 

 medium is precisely the same as it was when in air, while the 

 effect of the magnet, from whose interior the magnetic sub- 

 stance is of necessity excluded, is still - th of what it was in 



air. This latter seems to be the kind of experiment which 

 Mr. J. J. Thomson suggests in his June letter (p. 429), and 

 Phil. Mag. S. 5. Vol. 14. No. 89. Nov. 1882. 2 B 



