1. Sharp points;
2. Air gaps;
3. Sharp turns.
Lightning Rods.—This form of arrester consists of a conducting rod or cable erected on the outside of a building and connected to earth, in order to afford protection from lightning by carrying the lightning discharge into the ground; or to prevent lightning by leading the electricity from the earth to the cloud without disturbance.
Ques. Why do lightning rods terminate in sharp points?
Ans. The action of the rod depends on the discharging effect of a sharp point as follows: When an electrically charged cloud approaches a building provided with a lightning rod, it induces an opposite charge in the earth and in the rod which is connected to the earth. As soon as the charge on the point becomes strong enough to break apart the molecules of the air in front of it, a stream of electrified particles, opposite in sign to that of the charge on the cloud, passes from the neighborhood of the rod to the cloud and thus neutralizes the charge of the cloud.
Ques. How should a lightning rod be erected?
Ans. The conductor should be carried to all high points of the building it is to protect and should be well insulated from the latter and grounded in deep wet earth, independent of gas or water pipes. Sharp bends and corners should be avoided.
Fig. 1,039.—Diagram showing principle of air gap arrester: lightning discharges more readily at sharp points than along flat surfaces.
Ques. Why have lightning rods fallen into disfavor?