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of positively and iiegtitively cluirged particles with a total eliai-ge 

 zero, the positive charge of the positive particles being ex^ictly 

 equal to the negative charge of ihe negative ones. When we put 

 this piece of metal into motion we must ascribe to it a certain 

 amount of electromagnetic mass which is equal to the sum of the 

 electromagnetic masses of the positive and negative particles. If, 

 however, we send a curi-ent through the piece of metal, then the 

 energy of this current is not equal to \^ mv\ m being the mass 

 of a particle and v the mean velocity which is imparted to then) 

 by the electromotive force. 



This difference can be explained in the following way. In the 

 case that the piece of metal moves, the positive and negative particles 

 move in the same direction and then in all points of space as well 

 the eiecti'ical as the magnetical forces exerted by the different 

 electrons haNC a ditferent direction and nearly cancel each other, 

 in such a way that we tind only sensible forces in the points which 

 are so near one of the electrons that the forces exercised by that 

 electron strongly preponderate oxer those exercised by all the other 

 electrons, and need only to be taken into account. In the case that 

 a current passes through the metal on the other hand the magnetic 

 forces of a great pai-t of the electrons will act in the same direction, 

 and in a point at some distance, where the force .p exerted by a 

 single electron is negligibly small the magnetic force exercised by 

 nJ/ the electrons contained in a unit of volume of the metal (which 

 number we will call ^Vj together will nearly amount to ^Vp, in 

 consequence of which the energy will be of the order ^V^'p". This 

 energy pro\es not at all to be equal to the sum, but rather to N 

 times the sum of the amounts of energy which the single electrons 

 would occasion at that point. From this we may deduce that the 

 energy of the current is much larger than è^ mv". 



Though it might be worth while to try and calculate the amount 

 of this energy more accurately, it seems to me that there can be 

 little doubt but we should find for it: 



where L represents the coefficient of self-induction as it is usually 

 calculated from the magnetic energy alone, and 



If we assume that the current is transferred by only one kind of 

 electrons then we may write for ^^ mv' for each unit of volume 



If we now assume the "piece of metal" to be a circular circuit 



