362 Dr. 0. J. Lodge on the Dimensions of a Magnetic 



which, he says, Mr. Sargant then intended (and I hope still 

 intends) to carry out. 



The solenoid being now completely surrounded with homo- 

 geneous magnetic substance, wall a portion of it in with paper 

 or glass to the shape of the steel of the magnet, and then pull 

 both magnet and solenoid out into the air again. Naturally 

 the solenoid will still be /x times too strong for the magnet, 

 but no further discrepancy need be expected ; and if the cur- 

 rent of the solenoid has been weakened when inside the mag- 

 netic medium so as to restore the disturbed equivalence, they 

 will remain equivalent when the region external to both is 

 again filled with common air. 



These statements, if in their essence granted, require to 

 make them complete certain provisos about the boundary of 

 the vessel containing the magnetic medium, unless it be infi- 

 nitely large, and also a discussion of what happens in the case 

 of more than one magnetic medium. But the magnetization 

 of bounding surfaces, and the accidents which happen to lines 

 cutting surfaces of discontinuity, are perfectly understood and 

 need not be here entered into. 



Moreover, in making these statements I am merely saying 

 what one would expect to happen without evidence to the con- 

 trary; but I am not for an instant implying that direct experi- 

 mental investigation is unnecessary and would not be highly 

 desirable. On the contrary, I think it would be most desirable 

 and satisfactory to have the matter thoroughly sifted. 



Supposing, then, that I have so far made no mistake, we 

 can make the general statement of the equivalence of a cur- 

 rent and a magnet thus — 



The magnetic moment of a circuit is equal to the strength of 

 its current multiplied by its effective area and again multiplied 

 by the magnetic inductive capacity {or permeability) of the medium 

 in the interior of the region enclosed by the contour (which region 

 for a simple pAane circuit is a mere shell), but is wholly inde- 

 pendent of the magnetic properties of all the rest of the sur- 

 rounding medium. 



The corresponding dimensional equation is 



[mJ^L/^T- 1 ]. 



Substituting in this the value of [wi] from (1), we obtain the 

 relation 



W&]=£i (3) 



whence 



