What is called statical electricity is the condition of the subtle force in a state of electrical quiescence; and all electricity in motion, however excited by friction, heat, chemical action, or otherwise, is dynamic.
Perpetual modifications are taking place in electrical condition of all matter, that when made apparent, at first may seem quite inexplicable. The excited currents flow with amazing rapidity. Their actions and re-actions baffle our calculations, and the imagination itself is bewildered by their extent and complexity. Yet by electrical tests and laboratory experiments, carefully employed, the laws of electricity are now as well known as those of any other branch of physical science, and the phenomena, if more startling, are no more mysterious than the manifestations of heat, light and gravitation.
Atmospheric electricity is not different in kind from that brought into evidence by the methods of the experimenter in the laboratory, subject to his control, and much used in the arts and industries of life. The lightning that shineth from the one part under heaven to the other part under heaven, a bright light in the cloud, is the same as the electric spark from the moderately charged receiver, when the positive and negative poles are brought into contact—the same as the less intense spark excited by passing the hand rapidly over the fur on the cat’s back when the electrical conditions are favorable.
The storm cloud is a vast receiver and by induction becomes at times highly charged with electricity. If the cloud is at rest, and the heated air grows moist, that which is known as sheet or heat lightning appears in frequent flashes. The imprisoned electricity leaps forth from the bosom or edge of the cloud, but as instantly gathers itself back to its source, and apparently without tension or force enough to crash through the atmosphere to any distant object. The flashes are unaccompanied by the noise of thunder, and may be but reflections on the cloud from a source far beyond. We watch them without fear of danger, and the subdued impression is that of the beautiful.
Amidst the terrific grandeur of the violent thunder storm another form of lightning is seen; either the vivid flash that seems to envelop us, or zigzag, sometimes forked lines that dash across the cloud earthward, and occasionally, as in a return stroke, from the earth to the cloud.
In about the middle of the eighteenth century the identity of lightning with electricity was fully ascertained, and since then the most sublime and startling phenomena of our thunder storms are better understood. Under certain contingencies they must occur. Since the different clouds or portions of the same cloud are charged with different electricities, positive and negative, when these by the winds are brought near each other, or rolled together, fierce explosions follow, and great electrical changes take place in the clouds. Vast supplies of the imprisoned fiery fluid leap from strata to strata, or, if the distance is not too great, and the earth is at the same time strongly electrified, crash down to it through whatever sufficient conductors are found. If those not sufficient to receive and convey the charge be in the path they are dashed aside; men and beasts are killed by the shock, trees and other less perfect conductors are scattered in fragments.
Usually the more prominent objects as masts of ships, trees, and buildings are struck in the lightning’s course from the cloud, but occasionally those lowest down, near trees, and even in cellars receive the shock. In these cases the current is probably from the earth, whose electric condition is negative with respect to the clouds that pass over it. In either case the opposite electricities that strongly attract each other, and whose concurrence produces the destructive discharge near the earth’s surface are held apart by the stratum of air between them. When the attraction becomes too strong to be resisted by the insulating medium they rush together, in their fiery embrace, the flash and concussion being in proportion to the intensity of the charge.
Do lightning rods protect? Yes; but not perfectly. If properly constructed, and of sufficient conducting capacity, they are a source of safety, and to discard them as useless is not wise.
The instances in which buildings provided with rods have been struck do not prove them useless; or, as some say, that the rods do harm by attracting the lightning that they are unable to conduct to the earth without injury to the building. The point does not attract, but only catches the electricity that sweeps over it. When violent shocks or explosions occur the rod may be of little service. Its office is to prevent these by silently conducting the excess of electricity from the air. The rod, rightly placed, conducts to the earth all it can, lessening the evil it does not entirely prevent. But all danger is not removed. The position of the opposite poles in the immense battery may be such as to give the stroke a horizontal direction, and far below the point of the rod; such currents have been known to pass long distances through atmosphere and smite with destructive violence objects lying in their path. Against these lateral attacks rods above our roofs are probably little or no protection. Still the more good conductors there are in any locality the less danger, as they prevent the accumulation of electricity.
