Page:Electricity (1912) Kapp.djvu/59



the first chapter we investigated in a general way the force acting between two bodies, and we found that this force may be expressed by a mathematical formula which is the same for real masses, electricity and magnetism. The units as regards length, time and force may be the same in all cases, but the units in which we express the amount of active matter producing the force must naturally be different in each case. We have also seen that the nature of the medium across which the force acts is immaterial in the case of gravitation, but may modify the force in the case of electricity and magnetism. Let us now, without for the moment specifying any particular kind of active matter, assume that one of the bodies contains a large amount of active matter, say $$M$$ units, and that the other contains one unit only. For brevity I shall call them the large and the small body. We can then use the small body to