Ans. On account of numerous failures due to faulty installations, and non-maintenance of the rod in good condition, also because of the excessive prices charged by unscrupulous dealers for rods and their erection.
A lightning rod with defective insulation or broken ground connection is a danger rather than a protection.
Air Gap Arresters.—Many of the lightning arresters used for the protection of electrical apparatus depend upon the fact that lightning discharges will jump across air spaces that are good insulators for the regular working current, while they find difficulty in passing through circuits containing electromagnets.
Fig. 1,040.—Union lightning arrester and ground wire switch for telegraph lines. Two line wires are attached to the two plates provided with points. The ground wire being connected to the third or central plate. The pin serves as a ground wire switch and cut out. This is a good form for short lines.
The principle of air gap arresters is illustrated in [fig. 1,039]. There are two brass plates slightly separated; one is connected to the line and the other is grounded. The air gap between the plates is very small and the resistance thus interposed, while sufficient to prevent the regular working current jumping across, is not great enough to interfere with a lightning discharge which readily jumps the gap and passes off to earth.
Ques. Why are teeth provided on the plates?
Ans. For the same reason that points are used on lightning rods. That is, when electricity at high pressure accumulates at such points the surrounding air is electrified and the charge escapes by means of the charged air particles.