Lightning

Lightning is a large electric spark. In 1752 Franklin of Philadelphia discovered that thunderclouds were always electrified. This he did by sending a kite up into the cloud, and conducting the electricity down the string, and from this experiment he conceived the idea of protecting buildings by wire conductors, down which the electricity could be carried safely away. How it is that the clouds get electrified is not thoroughly known, but it is generally supposed that a certain amount of electricity is produced by friction between the lower clouds, or mists, and the rocks against which they are driven. When rain frequently falls, the charge is carried down, but after dry weather the charge accumulates, and lightning results. The flash takes place either between two clouds, or a cloud and the earth, the air really giving way under the electric strain. The break occurs at the weakest spot, and this will be where some high point on the earth is stretching out towards the clouds; forked lightning then occurs. Another flash often immediately follows the first, and this is known as the "back stroke." "Summer lightning" is generally due to a storm a long distance away. "Globe lightning," or "fire-ball," is not quite understood; it generally lasts several seconds, is less brilliant than forked lightning, and often bursts on reaching the ground, discharging flashes of lightning. The brightness of lightning is very great, but so quick is the flash that it is not fully observed; if it could be made to last even one-tenth of a second, it would be found to be 100,000 times as bright as moonlight. The curious odour noticed during a thunderstorm is due to the formation of ozone by the electric discharge.