534 Prof. J. C. Maxwell on a Dynamical Theory [Dec. 8, 



If this number is altered, either by motion of the curve, or motion of 

 the inducing current, or variation in its strength, an electromotive force acts 

 round the curve and is measured by the decrease of the number of lines 

 passing through it in unit of time. 



If the curve itself carries a current, then mechanical forces act on it tend- 

 ing to increase the number of lines passing through it, and the work done 

 by these forces is measured by the increase of the number of lines multi- 

 plied by the strength of the current. 



A method is then given by which the coefficient of self-induction of any 

 circuit can be determined by means of Wheatstone's electric balance. 



The next part of the paper is devoted to the mathematical expression of 

 the electromagnetic quantities referred to each point in the field, and to the 

 establishment of the general equations of the electromagnetic field, which 

 express the relations among these quantities. 



The quantities which enter into these equations are : — Electric currents 

 by conduction, electric displacements, and Total Currents ; Magnetic 

 forces. Electromotive forces, and Electromagnetic Momenta. Each of 

 these quantities being a directed quantity, has three components ; and be- 

 sides these we have two others, the Free Electricity and the Electric Poten- 

 tial, making twenty quantities in all. 



There are twenty equations between these quantities, namely Equations 

 of Total Currents, of Magnetic Force, of Electric Currents, of Electro- 

 motive Force, of Electric Elasticity, and of Electric Resistance, making 

 six sets of three equations, together with one equation of Free Electricity, 

 and another of Electric Continuity. 



These equations are founded on the facts of the induction of currents as 

 investigated by Faraday, Felici, &c., on the action of currents on a magnet 

 as discovered by Oersted, and on the polarization of dielectrics by electro- 

 motive force as discovered by Faraday and mathematically developed by 

 Mossotti. 



An expression is then found for the intrinsic energy of any part of the 

 field, depending partly on its magnetic, and partly on its electric polari- 

 zation. 



From this the laws of the forces acting between magnetic poles and 

 between electrified bodies are deduced, and it is shown that the state of 

 constraint due to the polarization of the field is|such as to act on the bodies 

 according to the well-known experimental laws. 



It is also shown in a note that, if we look for the explanation of the force 

 of gravitation in the action of a surrounding medium, the constitution of 

 the medium must be such that, when far from the presence of gross matter, 

 it has immense intrinsic energy, part of which is removed from it wherever 

 we find the signs of gravitating force. This result does not encourage us 

 to look in this direction for the explanation of the force of gravity. 



The relation which subsists between the electromagnetic and the electro- 

 static system of units is then investigated, and shown to depend upon what 



