Page:Electricity (1912) Kapp.djvu/104

100 surface is curved there is such a component, and it becomes the greater the more sharply the surface is curved. At a sharp corner it becomes very great, and if the corner is drawn out into a sharp point the force is so great that all the charge is dissipated as soon as brought to the conductor. Hence lightning-rods, which are intended to dissipate any charge which may be induced in a building by a charged cloud overhead as quickly as possible, and so avert the threatened stroke, are provided with sharp points. The gilding is not essential, but more in the nature of an extravagant refinement. The only excuse one can find for such a refinement is that gilding protects the iron from rusting, and so preserves the sharpness of the point. In the water-dropping machine we would, of course, also avoid the sharp corners by making the outside of each part more or less spherical, without, however, altering the inner and essential parts.

The connection between Figs. 2 and 3 will be obvious at a glance. The cylinder A corresponds to the charged sphere, and the drop of water hanging from the end of the pipe corresponds to the right-hand end of the cylinder. It becomes negatively electrified by induction, and on falling carries this