arise. We can so devise the conditions of the experiment that no other force than that acting between the large and the small body is present. The condition that one body should contain a charge large in comparison with the other is not essential, only convenient, as it obviates the necessity of making mathematical corrections which would be necessary if the small body contained a charge of the same order of magnitude. In this case the assumption that the charge distributed over the surface of a sphere acts in the same way as if it were concentrated in the centre is no longer strictly true for small distances, so that certain corrections become necessary.
The first physicist who investigated quantitatively the action of electric and magnetic forces across space was Coulomb, who towards the end of the eighteenth century invented for this purpose an instrument known as the torsion balance. As applied to electric measurements, it consists essentially of a very light scale beam made of sealing-wax and glass fibres, and suspended horizontally from a thin wire attached to its middle. One end of the beam carries a gilded pith ball, and the other a mica disc as a counterweight. The
