their centres will be greater than the distance between the points of suspension. If the charges are of opposite sign, the opposite will be the case. This shows respectively that a repulsive force and an attractive force is causing the deviation from the vertical. If we know the weight of the spheres, measure their distance and the angle of deviation of the suspending threads from the vertical, the force acting between the two charges can be calculated from well-known mechanical principles in quite a simple manner.
It is, however, necessary to avoid disturbing influences. The spheres must hang in the middle of a very large room, so that floor, ceiling and walls are far removed, and we must make the observations by telescope, as otherwise the presence of the body of the observer near the spheres would disturb the electrical equilibrium. I need hardly say that such an experiment would be expensive and difficult; in reality it need not be made, as there are other far more practical methods of investigation available, but it is convenient to imagine such an experiment, because it will enable me to explain in the simplest possible way certain first principles. Suppose then that we are not deterred by questions of
