P CONNECTION OF ELECTRICITY AND MAGNETISM. 261 



stick together, and more filings attach themselves to the exposed poles : 

 that is, to the ends of the row of filings. In this way the filings, instead 

 of forming a confused system of dots over the jjaper, draw together, 

 filing to filing, till long fibers of filings are formed, which indicate by 

 their direction the lines of force in every part of the field. 



The mathematicians saw in this experiment nothing but a method of 

 exhibiting at one view the direction in different places of tbe resultant 

 of two forces, one directed to each pole of the magnet, a somewhat com- 

 plicated result of the simple law of force. 



But Faraday, by a series of steps as remarkable for their geometrical 

 definiteness as for their speculative ingenuity, imparted to his concep- 

 tion of these lines of force a clearness and precision far in advance of 

 that with which the mathematicians could then invest their own formulas. 



In the first place, Faraday's lines of force are not to be considered 

 merely as individuals, but as forming a system, drawn in space in a 

 definite manner, so that the number of the lines which pass through an 

 area, say of one square inch, indicates the intensity of the force acting 

 through the area. Thus the lines of force become definite in number. 

 The strength of a magnetic pole is measured by the number of lines 

 which proceed from it ; the electro-tonic state of a circuit is measured 

 by the number of lines which pass through it. 



In the second place, each individual line has a continuous existence in 

 space and time. When a piece of steel becomes a magnet, or when an 

 electric current begins to flow, the lines of force do not start into exist- 

 ence, each in its own place, but, as the strength increases, new lines are 

 developed within the magnet or current, and gradually grow outward, 

 so that the whole system expands from within, like Newton's rings in 

 our former experiment. Thus, every line of force preserves its identity 

 during the whole course of its existence, though its shape and size may 

 be altered to any extent. 



I have no time to describe the methods by which every question relat- 

 ing to the forces acting on magnets or on currents, or to the induction 

 of currents in conducting circuits, may be solved by the consideration 

 of Faraday's lines of force. In this place they can never be forgotten. 

 By means of this new symbolism, Faraday defined with mathematical 

 precision the whole theory of electro -magnetism, in language free from 

 mathematical technicalities, and applicable to the most complicated as well 

 asthesimplestcases. ButFaraday didnotstop here. He went on from the 

 conception of geometrical lines of force to that of physical lines of force. 

 He observed that the motion which the magnetic or electric force tends 

 to produce is invariably such as to shorten the lines of force, and to 

 allow them to spread out laterally from each other. He thus perceived 

 in the medium a state of stress, consisting of a tension, like that of a rope. 

 in the direction of the lines of force, combined with a pressure in all 

 directions at right angles to them. 



This is quite a new conception of action at a distance, reducing it to 



