536 A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



proposed by him, is the same in substance as that which I have begun to 

 develope in this paper, except that in 1846 there were no data to calculate 

 the velocity of propagation. 



(21) The general equations are then applied to the calculation of the coef- 

 ficients of mutual induction of two circular currents and the coefficient of self- 

 induction in a coil. The want of uniformity of the current in the different 

 parts of the section of a wire at the commencement of the current is investi- 

 gated, I believe for the first time, and the consequent correction of the coefficient 

 of self-induction is found. 



These results are applied to the calculation of the self-induction of the coil 

 used in the experiments of the Committee of the British Association on Standards 

 of Electric Resistance, and the value compared with that deduced from the 

 experiments. 



PART II. 



ON ELECTROMAGNETIC INDUCTION. 



Electromagnetic Momentum of a Current. 



(22) We may begin by considering the state of the field in the neigh- 

 bourhood of an electric current. We know that magnetic forces are excited in 

 the field, their direction and magnitude depending according to known laws 

 upon the form of the conductor carrying the current. When the strength of 

 the current is increased, all the magnetic effects are increased in the same pro- 

 portion. Now, if the magnetic state of the field depends on motions of the 

 medium, a certain force must be exerted in order to increase or diminish these 

 motions, and when the motions are excited they continue, so that the effect 

 of the connexion between the current and the electromagnetic field surrounding 

 it, is to endow the current with a kind of momentum, just as the connexion 

 between the driving-point of a machine and a fly-wheel endows the driving-point 

 with an additional momentum, which may be called the momentum of the fly- 

 wheel reduced to the driving-point. The unbalanced force acting on the driving- 

 point increases this momentum, and is measured by the rate of its increase. 



