in the Motion of the Magnetic Field. 329 



important to examine the signification of the integral 



which represents, as is known, the area of a closed curve of 

 magnetization. 



I 



bhrwhS 



■■jppsaj 



IHHi 



If we write the equation (11) in the form 



4tt/2 \dl dJ'_ dR 

 3 U )~dt +X dt ~"W 

 dR 



the term — will represent the force by which the exterior 



magnetic field acts on the coordinate, the velocity of which 

 is I. In the time dt it is obvious that this force will per- 

 form the work 



dW=- ~Adt=-IdR. 



dt 



Therefore during the complete cycle of magnetization the 

 magnetic field will perform the work 



W=-jIdH, 



where the integral is extended along the curve of magneti- 

 zation. 



On the other hand, if we differentiate equation (26), multiply 

 it by J, and substitute for J its expression from (25), then we 



