1885.] 



Induction in the surrounding Field. 



1(39 



consists in the supposition that the electric and magnetic inductions 

 are transferred through the medium in a similar manner to the 

 energy, the induction being regarded as propagated sideways rather 

 than along the lines or tubes of induction. The tubes of electric 

 induction move in npon the wire containing a current, and are there 

 broken up or dissolved, while the magnetic tubes, which are ring- 

 shaped, contract upon the wire and finally disappear, the places of 

 the tubes thus lost being supplied by fresh tubes sent out from the 

 seat of electromotive force. 



Examining the basis of Maxwell's electromagnetic theory, it is seen 

 to rest on three principles, viz. : — 



I. The electric and magnetic energies are distributed through the 

 field in a manner which can be assigned. Dividing the unit tubes 

 into unit cells by level surfaces- drawn at unit differences of potential, 

 each electrical cell, according to Maxwell, contains half a unit of 



energy, and each magnetic cell -i- unit. 



II. The line integral of the electric intensity round any closed 

 curve is eqnal to the rate of decrease of the total magnetic induction 

 through the curve. 



III. The line integral of the magnetic intensity round any closed 

 curve, is equal to 47r x the current through the curve. 



The second and third principles are modified in a manner suggested 

 by the supposed movement of the tubes of induction. 



II modified. Whenever electromotive force is produced by a change 

 in the magnetic field or by motion of matter through the field, the 

 E.M.F. per unit length, or the electric intensity,, is equal to the 

 number of tubes of magnetic induction, cutting or cut by the unit 

 length per second, the E.MJ?. tending to produce induction in the 

 direction in which a right-handed screw would move if turned round 

 from the direction of motion, relatively to the tubes, towards the 

 direction of the magnetic induction. 



This will give the result of II above, if the quantity of magnetic 

 induction through a curve only alters by the movement of tubes in or 

 out across the boundary. Reasons for the supposition are given. 



III modified. Using the term magnetomotive force, a term sug- 

 gested by Mr. Bosanquet, to denote the line integral of the magnetic 

 intensity round the axis of a tube of induction, the third principle is 

 as follows : — 



Whenever magnetomotive force is produced by change in the electric 

 field, or by motion of matter through the field, the magnetomotive 

 force per unit length is equal to 47rx the number of tubes of electric 

 induction cutting or cut by unit length per second, the magnetomotive 

 force tending to produce induction in the direction in which a right- 

 handed screw would move if turned round from the direction of the 



VOL. XXXVIII. 



