554 PROPERTIES OF THE ELECTROMAGNETIC FIELD. 



572. EQUATIONS OF THE ELECTRICAL FIELD. Let us consider, 

 lastly, an isotropical field in which continuous currents may 

 coexist with currents corresponding to a variation in the electrical 

 displacement (126). 



If /> & & are the components of the electrical displacement, the 

 true quantities of electricity u', v', w\ which in unit time traverse unit 

 surface perpendicularly to the axis, consist of continuous currents 

 #, v, w (569), and of variations of electrical displacement. We 

 have then 



(II) *-" + 



The equations of the currents (4) become 



ax 



When the medium is a dielectric, it establishes equilibrium 

 between the electromotive force, whose components are given by 

 equation (10), and the elastic reactions developed by the displace- 

 ment. If is the value in electromagnetic units of the coefficient 

 K. 



of electrical elasticity that is to say, the ratio of the electromotive 

 force to the displacement we shall have 



