560 A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



force is perpendicular to the direction of motion and to the lines of magnetic 

 force ; and if a parallelogram be drawn whose sides represent in direction 

 and magnitude the velocity of the conductor and the magnetic induction at that 

 point of the field, then the area of the parallelogram will represent the electro- 

 motive force due to the motion of the conductor, and the direction of the force 

 is perpendicular to the plane of the parallelogram. 



The second term in each equation indicates the effect of changes in the 

 position or strength of magnets or currents in the field. 



The third term shews the effect of the electric potential $. It has no effect 

 in causing a circulating current in a closed circuit. It indicates the existence 

 of a force urging the electricity to or from certain definite points in the field. 



Electric Elasticity. 



(66) When an electromotive force acts on a dielectric, it puts every part 

 of the dielectric into a polarized condition, in which its opposite sides are 

 oppositely electrified. The amount of this electrification depends on the electro- 

 motive force and on the nature of the substance, and, -in solids having a structure 

 defined by axes, on the direction of the electromotive force with respect to these 

 axes. In isotropic substances, if k is the ratio of the electromotive force to the 

 electric displacement, we may write the 



Equations of Electric Elasticity, 



(E). 



Electi-ic Resistance. 



(67) When an electromotive force acts on a conductor it produces a current 

 of electricity through it. This effect is additional to the electric displacement 

 already considered. In solids of complex structure, the relation between the 

 electromotive force and the current depends on their direction through the solid. 



