ing out of such an experiment would be very difficult and costly. As I am, however, not concerned with any actual experiment, but with the explanation of first principles, I prefer to base this explanation on the mathematically simple but technically impracticable experiment rather than on an experiment which, although easy to perform, requires a complicated mathematical interpretation.
Let us then revert to the hypothetical experiment of a large electrified sphere suspended by a silk thread in the middle of a large room. Let the sphere contain a charge of electrostatic units of positive electricity. Let the small sphere contain unit positive charge, and let us assume that we may place this at, and measure accurately the force at, any point. We shall then find that the general law of action through space holds good if we measure distances between the centres of the spheres. This means that the distributed charge on each sphere acts as if it were concentrated in its centre, a fact which can also be proved mathematically, starting from the general law. The repelling force on unit charge is given in dynes by the expression
