478 
PROFESSOR CLERK MAXWELL OX THE ELECTROMAGNETIC FIELD. 
Then, if sliding the moveable conductor in a given direction increases the value of M, 
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 direction 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 conductor 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 sur- 
face 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 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 completely 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 
