138 A COMPARISON OF THE ELECTRIC UNITS 



Now let A and B be kept asunder by a rigid rod. 



The combined system, if set in motion in the direction AB, will pull in 

 that direction with a force which may either continually augment the velocity, 

 or may be used as an inexhaustible source of energy. 



I think that these remarkable deductions from the latest developments of 

 Weber and Neumann's theory can only be avoided by recognizing the action of 

 a medium in electrical phenomena. 



The statement of the electromagnetic theory of light in my former paper 

 was connected with several other electromagnetic investigations, and was there- 

 fore not easily understood when taken by itself. I propose, therefore, to state 

 it in what I think the simplest form, deducing it from admitted facts, and 

 shewing the connexion between the experiments already described and those 

 which determine the velocity of light. 



The connexion of electromagnetic phenomena may be stated in the following 

 manner. 



THEOREM A. If a closed curve be drawn embracing an electric current, 

 then the integral of the magnetic intensity taken round the closed curve is 

 equal to the current multiplied by 4ir. 



The integral of the magnetic intensity may be otherwise defined as the 

 work done on a unit magnetic pole carried completely round the closed curve. 



This well-known theorem gives us the means of discovering the position 

 and magnitude of electric currents, when we can ascertain the distribution of 

 magnetic force in the field. It follows directly from the discovery of (Ersted. 



THEOREM B. If a conducting circuit embraces a number of lines of mag- 

 netic force, and if, from any cause whatever, the number of these lines is 

 diminished, an electromotive force will act round the circuit, the total amount 

 of which will be equal to the decrement of the number of lines of magnetic 

 force in unit of time. 



The number of lines of magnetic force may be otherwise defined as the 

 integral of the magnetic intensity resolved perpendicular to a surface, multiplied 

 by the element of surface, and by the coefficient of magnetic induction, the 

 integration being extended over any surface bounded by the conducting circuit. 



This theorem is due to Faraday, as the discoverer both of the facts and 

 of this mode of expressing them, which I think the simplest and most com- 

 prehensive. 



