Permanent Magnets. 149 



by a small coil carrying a current, produces a magnetic short 

 circuit through the surrounding medium, instead of contribu- 

 ting in full measure to strengthening the general distribution. 

 However this may be, the broad features are that the total 

 moment of A, amounting to about 3500, is neutralized by the 

 action of a coil which would be measured by 129, or some- 

 what less than the disk-moment*. It is clear that the 

 consideration of moments lends no convenient shape to the 

 problem. 



Consider instead the magnet as decomposed into a number 

 of thin disks, each of which can be represented according to 

 the fundamental convention of electromagnetism by a current 

 round its edge. Then the number of current-turns is repre- 

 sented by the quotient of the moment by the area. 



150 



In magnet A, — - — = 49*9 current-turns, 

 area 



= 499 ampere-turns. 

 Ampere-turns in demagnetizing coil = 430. 



Allowing, as above, for the discrepancy, we assume for the 

 present that the demagnetizing current-turns are equivalent 

 to those of the limiting disk-moment. 



If, therefore, we conceive of a permanent magnet as having 

 a magnetic potential imbedded in its substance of the same 

 nature as that which would be due to amperian currents, and 

 acting on the steel in the same manner as an external potential 

 produced by a coil, we have a convenient statement of the 

 facts. For the amperian currents, or the potential, have been 

 measured in two ways, viz. by cutting up into disks and by 

 neutralization with a coil ; and the results are accordant, 

 allowing for the imperfections and difficulty of the investi- 

 gation. 



I proceed to develop the mode of accounting for the fore- 

 going facts by Faraday's analogy of the voltaic circuit; that 

 is to say, by the application of an analogue of Ohm's law: — 



, T ,. . , Diff. of Potential 



Magnetic resistance = ^-. -, 



° Intensity 



I formerly called Difference of Potential, Magnetomotive 



* The radius of the coil being about 1*27 centim., its total moment was 

 about 218. But it is conceived that the space external to the magnet 

 cannot have exercised any considerable action on the needle, nor any on 

 the magnet. 



