Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/210

 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 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. 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 tube 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, so that the number of them which intersect a unit area placed at right angles to their direction