876 



SCIENCE. 



[N. S. Vol. II. No. 5% 



completely satisfactory manner by a me- 

 chanical model is of no material conse- 

 quence as far as the correctness of the laws 

 of flux and the definiteness of our ideas of 

 this statical element of electric and magnetic 

 force is concerned. 



We now come to the second essential fea- 

 ture of Maxwell's theory. It deals with 

 what may be called the dynamic element 

 of our modern ideas concerning electric 

 and magnetic force. Oersted discovered 

 that a conductor which is the seat of that 

 progressive process which we call an elec- 

 tric current is accompanied by magnetic 

 forces which are present in every element 

 of the space surrounding the conductor. 

 Ampere formulated the law in accordance 

 with which this force is distributed in 

 space. This law can be stated broadly as 

 follows : 



The magneto-motive force around the 

 boundary line of anj'^ elementary area is 

 proportional to the electric current, or what 

 is the same thing, to the rate of variation 

 of the electric flux through that area. 



This law is one of the fundamental laws 

 of the Faraday-Maxwell theory, but al- 

 though its form is essentially the same here 

 as it was in the old theories its mean- 

 ing is very much more comprehensive. In 

 the old theory the magneto-motive force 

 around the boundary of any elementarj' area 

 through which no conduction current passes 

 is always zero. According to the new view 

 the current is not confined to conductors, 

 but extends to the dielectric and its value 

 through any elementary area is equal to 

 the rate of variation of the electric flux or 

 integral cui-rent through that area. The 

 law of magneto-motive force just mentioned 

 applies to this current just as well as it does 

 to currents in conductors. Again, in the 

 old theory the magnetic force accompany- 

 ing an electric current was a direct action 

 at a distance between the various elements 

 of the conductor cai-rying the conduction 



current and a magnetic pole ; according to- 

 the new theory the magnetic force at any 

 point of the medium is the same in this case 

 as in an J' other, that is, a magneto-motive re- 

 action in the medium produced by the inte- 

 gral magnetic current, that is, by the mag- 

 netic flux or induction, which was set up in 

 the medium while the electric currents in 

 the various parts of the field increased from 

 their zero value to the value which they 

 have at the moment under consideration. 

 It must be observed, however, that since in 

 the law just mentioned the magnetic force 

 figures as a rate of change of the electric flux, 

 that this law presents to us the dynamic 

 element of the magnetic force just as New- 

 ton's second law of motion presents to us 

 the dynamic element of the mechanical 

 force. 



We proceed now to consider a similar 

 aspect of the electric force. Any change 

 in the electric currents brings with it a 

 change in the integral magnetic currents 

 in the various elements of the field, and 

 hence it implies work against the mag- 

 neto-motive reactions in those elements. 

 Hence, every electro-motive action tending 

 to change the electric currents in any part 

 of the field experiences a reaction to which 

 every element of the field contributes its 

 definite share, just as a change in the mo- 

 tion of any part of a mechanism is accom- 

 panied by a reaction to w'hich everj' other 

 part contributes its definite amount. How 

 does this reaction against a change of the 

 electric current manifest itself? The an- 

 swer to this momentous question was first 

 given by Faraday when he discovered the 

 magneto-electric induction. This discov- 

 ery can be described as follows: 



Consider a loop of a conducting wire and 

 a magnet, in its vicinity. A change of rel- 

 ative position of the two produces a current 

 in the loop. If the magnet is an electro- 

 magnet, and if we keep the relative position 

 unchanged and change the strength of the 



