Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/407

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314.] If there are spheres of radius and resistance placed in a medium whose resistance is at such distances from each other that their effects in disturbing the course of the current may be taken as independent of each other, then if these spheres are all contained within a sphere of radius the potential at a great distance from the centre of this sphere will be of the form


where the value of is


The ratio of the volume of the small spheres to that of the sphere which contains them is


The value of the potential at a great distance from the sphere may therefore be written


Now if the whole sphere of radius had been made of a material of specific resistance we should have had


That the one expression should be equivalent to the other,


This, therefore, is the specific resistance of a compound medium consisting of a substance of specific resistance in which are disseminated small spheres of specific resistance the ratio of the volume of all the small spheres to that of the whole being In order that the action of these spheres may not produce effects depending on their interference, their radii must be small compared with their distances, and therefore must be a small fraction.

This result may be obtained in other ways, but that here given involves only the repetition of the result already obtained for a single sphere.

When the distance between the spheres is not great compared with their radii, and when is considerable, then other terms enter into the result, which we shall not now consider. In consequence of these terms certain systems of arrangement of