330 A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



we have some reason to suppose that this motion is one of rotation, having 

 the direction of the magnetic force as its axis. 



(9) We may now consider another phenomenon observed in the electro- 

 magnetic field. When a body is moved across the lines of magnetic force it 

 experiences what is called an electromotive force ; the two extremities of the 

 body tend to become oppositely electrified, and an electric current tends to flow 

 through the body. When the electromotive force is sufficiently powerful, and is 

 made to act on certain compound bodies, it decomposes them, and causes one 

 of their components to pass towards one extremity of the body, and the other 

 in the opposite direction. 



Here we have evidence of a force causing an electric current in spite of 

 resistance ; electrifying the extremities of a body in opposite ways, a condition 

 which is sustained only by the action of the electromotive force, and which, as 

 soon as that force is removed, tends, with an equal and opposite force, to 

 produce a counter current through the body and to restore the original electrical 

 state of the body ; and finally, if strong enough, tearing to pieces chemical 

 compounds and carrying their components in opposite directions, while their 

 natural tendency is to combine, and to combine with a force which can generate 

 an electromotive force in the reverse direction. 



This, then, is a force acting on a body caused by its motion through the 

 electromagnetic field, or by changes occurring in that field itself; and the eftect 

 of the force is either to produce a current and heat the body, or to decompose 

 the body, or, when it can do neither, to put the body in a state of electric 

 polarization, a state of constraint in which opposite extremities are oppositely 

 electrified, and from which the body tends to relieve itself as soon as the 

 disturbing force is removed. 



(10) According to the theory which I propose to explain, this "electro- 

 motive force " is the force called into play during the communication of motion 

 from one part of the medium to another, and it is by means of this force 

 tliat the motion of one part causes motion in another part. When electromotive 

 force acts on a conducting circuit, it produces a current, which, as it meets 

 with resistance, occasions a continual transformation of electrical energy into 

 heat, which is incapable of being restored again to the form of electrical energy 

 by any reversal of the process. 



