552 A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



that the value of M for every reticulation formed by the intersection of the 

 two systems of lines is unity. 



Finally, from every point of intersection of these reticulations let a line be 

 drawn through the field, always coinciding in direction with the direction of 

 magnetic force. 



(50) In this way the whole field will be filled with lines of magnetic force 

 at regular intervals, and the properties of the electromagnetic field will be com- 

 pletely expressed by them. 



For, 1st, If any closed curve be drawn in the field, the value of M for 

 that curve will be expressed by the number of lines of force which pass through 

 that closed curve. 



2ndly. If this curve be a conducting circuit and be moved through the 

 field, an electromotive force will act in it, represented by the rate of decrease 

 of the number of lines passing through the curve. 



3rdly. If a current be maintained in the circuit, the conductor will be 

 acted on by forces tending to move it so as to increase the number of lines 

 passing through it, and the amount of work done by these forces is equal to 

 the current in the circuit multiplied by the number of additional lines. 



4thly. If a small plane circuit be placed in the field, and be free to turn, 

 it will place its plane perpendicular to the lines of force. A small magnet will 

 place itself with its axis in the direction of the lines of force. 



Sthly. If a long uniformly magnetized bar is placed in the field, each pole 

 will be acted on by a force in the direction of the lines of force. The number 

 of lines of force passing through unit of area is equal to the force acting on 

 a unit pole multiplied by a coefficient depending on the magnetic nature of the 

 medium, and called the coefficient of magnetic induction. 



In fluids and isotropoic solids the value of this coefficient p, is the same 

 in whatever direction the lines of force pass through the substance, but in 

 crystallized, strained, and organized solids the value of p. may depend on the 

 direction of the lines of force with respect to the axes of crystallization, strain, 

 or growth. 



In all bodies p. is affected by temperature, and in iron it appears to diminish 

 as the intensity of the magnetization increases. 



