There is no sharp line of demarcation between insulators and conductors. Dry wood, for instance, is not a perfect insulator; and when damp it is not a perfect conductor. Dry air at atmospheric pressure is almost a perfect insulator, but if rarefied or at high temperature it becomes more or less of a conductor. All metals are conductors, but they are not all equally good conductors. Mercury is not so good a conductor as iron, iron is not so good as copper, and silver is still a slightly better conductor than copper. The difference between the two last-named metals is, however, not great enough to justify commercially the use of silver instead of copper wire in the construction of electrical machinery. For the present we need not inquire further into any fine gradations between conductors and insulators.
We assume that the silk threads used for the suspension of the charged spheres and the air surrounding them are perfect insulators, so that the spheres will retain their charges as long as they do not come into actual contact with each other or some other conductor. If we suspend the spheres near each other we find that they do not hang plumb. If they are both positively or both negatively charged the distance between
