=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. [Illustration: FIG. 203.--Electrical wind produced by a pointed conductor.] [Illustration: FIG. 204.--Electrical whirl. The reaction from the electrical wind causes it to revolve.] [Illustration: FIG 205. The wire screen protects the electroscope.] The charge in an electrified cloud acts inductively upon the earth beneath, attracting an opposite charge to the objects below. The discharge from the cloud often passes to the objects beneath, such as trees or buildings. _Thunder_ is believed to be due to the sudden expansion of the air when intensely heated by the electric discharge and its sudden contraction, like a _slap_, as the track instantly cools. Thunder at a distance is usually followed by rumblings due to changes in the intensity of the sound mainly due to reflections of sound waves from clouds and other reflecting surfaces. =226. An electric screen= is a device for cutting off the influence of an electric charge. Faraday found that if a sensitive electroscope is surrounded by a wire mesh screen (see Fig. 205), no evidence of electrification could be found inside. In other words, a network of conductors on a building makes the best protection against lightning, provided it is connected to the earth by good conductors at several places. Important Topics 1. Electrical theories. Evidences for electron theory. 2. How is the theory used in explaining induction? 3. Charges, and distribution on conductors (effect of shape). 4. Lightning: cause, effects, lightning rods. Exercises 1. In what respects is Franklin's one-fluid theory like the electron theory? In what respects different? 2. Consider two shells charged by induction from an electrified rubber rod, one positively and one negatively. Explain the process, using the ideas of the electron theory. 3. Should the metal top of an electroscope have sharp corners? Explain. 4. Would a tall steel tower have the same need of a lightning rod as a brick chimney of the same height? Explain. 5. Will a solid sphere hold a greater charge of electricity than a hollow one of the same diameter? Explain.