190 Faraday. 



These curves suggested to Faraday* the idea of lines of magnetic 

 force, or curves whose direction at every point coincides with 

 the direction of the magnetic intensity at that point; the 

 curves in which the iron filings arrange themselves on the 

 paper resemble these curves so far as is possible subject to the 

 condition of not leaving the plane of the paper. 



With these lines of magnetic force Faraday conceived all 

 space to be filled. Every line of force is a closed curve, which 

 in some part of its course passes through the magnet to which 

 it belongs, f Hence if any small closed curve be taken in space, 

 the lines of force which intersect this curve must form a 

 tubular surface returning into itself ; such a surface is called a 

 tiibe of force. From a tube of force we may derive information 

 not only regarding the direction of the magnetic intensity, 

 but also regarding its magnitude; for the product of this 

 magnitude} and the cross-section of any tube is constant along 

 the entire length of the tube. On the basis of this result, 

 Faraday conceived the idea of partitioning all space into 

 compartments by tubes, each tube being such that this product 

 has the same definite value. For simplicity, each of these 

 tubes may be called a " unit line of force " ; the strength of 

 the field is then indicated by the separation or concentration of 

 the unit lines of force,! I so that the number of them which 

 intersect a unit area placed at right angles to their direction 



#They were first defined in Exp. Res., 114 : "By magnetic curves, I mean 

 the lines of magnetic forces, however modified hy the juxtaposition of poles, 

 which could be depicted by iron filings ; or those to which a very small magnetic 

 needle would form a tangent." 



t Exp. Res. iii, p. 405. 



J Within the substance of magnetized bodies we must in this connexion under- 

 stand the magnetic intensity to be that experienced in a crevice whose sides are 

 perpendicular to the lines of magnetization : in other words, we must take it to be 

 what since Maxwell's time has been called the magnetic induction. 



Exp. Res., 3073. This theorem was first proved by the French geometer 

 Michel Chasles, in his memoir on the attraction of an ellipsoidal sheet, Journal 

 de 1'Ecole Polyt. xv (1837), p. 266. 



|| Ibid., 3122. "The relative amount of force, or of lines of force, in a 

 given space is indicated by their concentration or separation i.e., by their number 

 in that space." 



