554 A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



PART III. 

 GENERAL EQUATIONS OF THE ELECTROMAGNETIC FIELD. 



(53) Let us assume three rectangular directions in space as the axes of 

 x, y, and z, and let all quantities having direction be expressed by their com- 

 ponents in these three directions. 



Electrical Cuivents (p, q, r). 



(54) An electrical current consists in the transmission of electricity from 

 one part of a body to another. Let the quantity of electricity transmitted in 

 unit of time across unit of area perpendicular to the axis of x be called p, then 

 jj is the component of the current at that place in the direction of x. 



We shall use the letters p, q, r to denote the components of the current 

 per unit of area in the directions of x, y, z. 



Electrical Displacements (f, g, h). 



(55) Electrical displacement consists in the opposite electrification of the 

 sides of a molecule or particle of a body which may or may not be accom- 

 panied with transmission through the body. Let the quantity of electricity which 

 would appear on the faces dy . dz of an element dx, dy, dz cut from the body 

 be f .dy .dz, then f is the component of electric displacement parallel to x. We 

 shall use f, g, h to denote the electric displacements parallel to x, y, z respectively. 



The variations of the electrical displacement must be added to the currents 

 ft, q, r to get the total motion of electricity, which we may call p', q', r, so that 



.(A). 

 dh 



