26 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXVIII.) 
3073. A point equally important to the definition of these lines is, that they repre- 
sent a determinate and unchanging; amount of force. Though, therefore, their forms, 
as they exist between two or more centres or sources of magnetic power, may vary 
very greatly, and also the space through which they may be traced, yet the sum of 
power contained in any one section of a given portion of the lines is exactly equal to 
the sum of pow'^er in any other section of the same lines, however altered in form, or 
however convergent or divergent they may be at the second place. The experimental 
proof of this character of the lines will be given hereafter ( 3109 . &c.). 
3074. Now it appears to me that these lines may be employed with great advan- 
tage to represent the nature, condition, direction and comparative amount of the 
magnetic forces ; and that in many cases they have, to the physical reasoner at least, 
a superiority over that method which represents the forces as concentrated in centres 
of action, such as the poles of magnets or needles ; or some other methods, as, for 
instanee, that which considers north or south magnetisms as fluids diffused over the 
ends or amongst the particles of a bar. No doubt, any of these methods which does 
not assume too much, will, with a faithful application, give true results ; and so they 
all ought to give the same results as far as they can respectively be applied. But some 
may, by their very nature, be applicable to a far greater extent, and give far more 
varied results, than others. For just as either geometry or analysis may be employed 
to solve correctly a particular problem, though one has far more power and capa- 
bility, generally speaking, than the other ; or just as either the idea of the reflexion 
of images, or that of the reverberation of sounds maybe used to represent certain 
physical forces and conditions ; so may the idea of the attractions and repulsions of 
centres, or that of the disposition of magnetic fluids, or that of lines of force, be 
applied in the consideration of magnetic phenomena. It is the occasional and more 
frequent use of the latter which I at present wish to advocate. 
3075. I desire to restrict the meaning of the term line offeree, so that it shall 
imply no more than the condition of the force in any given place, as to strength and 
direction ; and not to include (at present) any idea of the nature of the physical cause 
of the phenomena ; or to be tied up with, or in any way dependent on, such an idea. 
Still, there is no impropriety in endeavouring to conceive the method in which the 
physical forces are either excited, or exist, or are transmitted ; nor, when these by 
experiment and comparison are ascertained in any given degree, in representing them 
by any method which we adopt to represent the mere forces, provided no error is thereby 
introduced. On the contrary, when the natural truth and the conventional repre- 
sentation of it most closely agree, then are we most advanced in our knowledge. 
The emission and the ether theories present such cases in relation to light. The idea 
of a fluid or of two fluids is the same for electricity ; and there the further idea of a 
current has been raised, which indeed has such hold on the mind as occasionally to 
embarrass the science as respects the true character of the physical agencies, and 
may be doing so, even now, to a degree which we at present little suspect. The 
