224. Distribution of an Electric Charge upon a Conductor.—We have applied the electron theory in explaining the phenomenon of electrostatic induction. Let us now use it in studying the distribution of an electric charge upon a conductor. Let a cylindrical metal vessel open at the top and insulated by being placed upon pieces of sealing wax have a charge of negative electricity given it. (See Fig. 201.) On now taking a proof plane and attempting to obtain a charge from the interior of the vessel no result is found, while a charge is readily obtained from the outside of the dish. This result is explained by considering that the electrons are mutually self-repellent and in their attempt to separate as widely as possible pass to the outer surface of the vessel. This same condition is also true of a dish made of woven wire. If the charged conductor is not spherical in outline, an uneven distribution of the charge is observed. Thus if an egg-shaped conductor is insulated and charged (see Fig. 202), a proof plane touched to the broad end of the body and then to an electroscope causes a certain divergence of the leaves of the latter. If now a charge be taken from the pointed end by the proof plane to the uncharged electroscope, a greater spreading of the leaves than before will be noticed. This indicates that the electricity may be unevenly distributed over the surface of a body. It is found that the electric density, as it is called, is greatest where the surface curves most sharply. At a very sharp curve, as at a point, the electric density may be so great that a part of the charge escapes into the air. (See Fig. 203.) For this reason electric conductors on which it is desired to keep an electric charge have round surfaces and all sharp points and corners are avoided. While conductors, such as lightning rods, which are designed to facilitate the escape of electric charges, are provided with a number of sharp points at the end or elsewhere. At such points, air particles are drawn forcibly against the point and after being charged are driven away strongly, creating the so-called electrical wind which carries away the charge at a rapid rate. (See Fig. 203.)
Fig. 201.—No charge is found inside a hollow vessel.
Fig. 202.—More charge at the pointed end.
225. Lightning and Electricity.—The fact that lightning is an electrical discharge was first shown in 1752 by Benjamin Franklin, who drew electric charges from a cloud by flying a kite in a thunderstorm. With the electricity which passed down the kite string he performed a number of electrical experiments. This discovery made Franklin famous among scientific men everywhere. Franklin then suggested the use of lightning rods to protect buildings from lightning. These rods act as conductors for the electric discharge and thus prevent it from passing through the building, with the risk of overheating some part and setting the latter on fire. The points provided at the top of lightning rods are believed to aid in preventing strokes of lightning by the silent discharge of the so-called electric wind which tends to quietly unite the charges in the clouds and on the earth beneath.
Fig. 203.—Electrical wind produced by a pointed conductor.
Fig. 204.—Electrical whirl. The reaction from the electrical wind causes it to revolve.
Fig 205. The wire screen protects the electroscope.