A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 551 



We may begin by supposing B to consist of a short straight conductor 

 with its extremities sliding on two parallel conducting rails, which are put in 

 connexion at some distance from the sliding-piece. 



Then, if sliding the moveable conductor in a given direction increases the 

 value of M t a negative electromotive force will act in the circuit B, tending 

 to produce a negative current in B during the motion of the sliding-piece. 



If a current be kept up in the circuit B, then the sliding-piece will itself 

 tend to move in that direction, which causes M to increase. At every point 

 of the field there will always be a certain direction such that a conductor moved 

 in that direction does not experience any electromotive force in whatever direc- 

 tion its extremities are turned. A conductor carrying a current will experience 

 no mechanical force urging it in that direction or the opposite. 



This direction is called the direction of the line of magnetic force through 

 that point. 



Motion of a conductor across such a line produces electromotive force in 

 a direction perpendicular to the line and to the direction of motion, and a con- 

 ductor carrying a current is urged in a direction perpendicular to the line and 

 to the direction of the current. 



(48) We may next suppose B to consist of a very small plane circuit 

 capable of being placed in any position and of having its plane turned in any 

 direction. The value of M will be greatest when the plane of the circuit is 

 perpendicular to the line of magnetic force. Hence if a current is maintained 

 in B it will tend to set itself in this position, and will of itself indicate, like 

 a magnet, the direction of the magnetic force. 



On Lines of Magnetic Force. 



(49) Let any surface be drawn, cutting the lines of magnetic force, and 

 on this surface let any system of lines be drawn at small intervals, so as to 

 lie side by side without cutting each other. Next, let any line be drawn on 

 the surface cutting all these lines, and let a second line be drawn near it, its 

 distance from the first being such that the value of M for each of the small 

 spaces enclosed between these two lines and the lines of the first system is 

 equal to unity. 



In this way let more lines be drawn so as to form a second system, so 



