55G A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



round the circuit, we shall get the total electromagnetic momentum of the circuit, 

 or the number of lines of magnetic force which pass through it, the variations 

 of which measure the total electromotive force in the circuit. This electromag- 

 netic momentum is the same thing to which Professor Faraday has applied the 

 name of the Electrotonic State. 



If the circuit be the boundary of the elementary area dtjdz, then its electro- 

 magnetic momentum is 



fdll d 



and this is the number of lines of magnetic force which pass through the 

 area dy dz. 



Magnetic Force (a, ft, y). 



(59) Let a, ft, y represent the force acting on a unit magnetic pole placed 

 at the given point resolved in the directions of x, y, and z. 



Coefficient of Magnetic Induction (/x). 



(GO) Let n be the ratio of the magnetic induction in a given medium to 

 that in air under an equal magnetizing force, then the number of lines of force 

 in unit of area perpendicular to x will be fia (p is a quantity depending on 

 the nature of the medium, its temperature, the amount of magnetization already 

 produced, and in crystalline bodies varying with the direction). 



(61) Expressing the electric momentum of small circuits perpendicular to 

 the three axes in this notation, we obtain the following 



Equations of Magnetic Force, 



m dH_dQ 



~ dy !: 



dF dH (B) 



dG dF 



