Page:The American Cyclopædia (1879) Volume X.djvu/472

 466 LIGHTNING ger. Their office is that of conductors of the electrical current, as the bed of a river presents itself for the flow of the aqueous current. Each may act as a safety valve to its respective cur- rent when this is impelled with unusual vio- lence ; and in case of obstruction to either dis- astrous consequences may ensue. Iron rods loosely jointed. together, and perhaps rusty in the joints, furnish a bad conveyance for the electrical current ; and if not continued down into moist ground, and there branching out, the passage of- the electricity into the earth may not be so free as by other conductors in the build- ing itself. Wrought-iron rods are commonly used in the United States on account of their greater cheapness. They should be at least three fourths of an inch in diameter, and in as long pieces as is practicable. The joints that cannot be avoided should be very securely fit- ted, so that the two ends are brought into close contact, and touch each other for several inches in length. The branching terminations in the ground may very well be filled around with charcoal, which is a good conductor, and also protects the rod to some extent from rusting. The points at the top may be protected from rust by gold leaf, and the whole rod may be painted with black paint having lampblack for its chief ingredient. A good rod may be se- cured without danger to the building by wooden clamps with iron fastenings, or even with iron staples. Glass insulators are useless, for when wet they become conductors. It is recommend- ed by some persons, that as the greatest number of thunder storms in this country come from the northwest, the conductors should be placed on the side of the building exposed to their first approach. But it is particularly important that every prominent elevated point of a large build- ing should be protected by its own rod, and it is well to connect all the rods together, and to have two or more stems running into the ground. It is very uncertain how large an area a rod of given height can protect. Differ- ent French electricians have variously rated it as a circular space of radius from one to three times the height of the rod above the highest point to which it is attached ; but little confi- dence can be placed in these conclusions. The opposite electricities, the concurrence of which produces the discharge, are far from being uniformly distributed through the atmosphere, and their point of rushing together may not be in any way under the influence of a rod directed into the air in its vicinity. The posi- tion of the excited masses may be favorable for a lateral discharge, and such have been known to pass horizontally through the atmosphere long distances, and to strike with destructive violence objects lying in their path. And as evidence of the protecting influence of a single point not reaching to any considerable distance, a case is cited of the foremast of a ship being struck, causing serious damage to the vessel, when the mainmast was provided with a con- ductor. Hence the importance of points upon the rods along the salient parts of buildings they are designed to protect. By the great multiplication of conductors the accumulation of opposite electricities in quantities sufficient to produce destructive discharges is prevented ; and thus it is that houses in cities are rarely struck, or vessels where many are lying to- gether in the docks. Isolated houses are more commonly the objects of the lightning stroke ; and it is observed that particular localities are subject to be repeatedly struck at different periods ; other spots are singularly free from such visitations. Chimneys from which hot and rarefied air is ascending into the atmos- phere, and barns stored with new hay, the vapors from which also produce warm ascend- ing currents, are especially liable to be struck. It is prudent for persons in a building to avoid being near a chimney or the walls, or in close proximity to metallic bodies, along which the lightning may find the readiest path. The greatest safety would be found, as stated by Franklin, in lying in a hammock suspended by silken cords in the middle of a large apartment. Insulation by placing one's self upon a feather bed, or any poor conductor, is also a protec- tion, not however complete unless the head is covered by some non-conducting substance. The efficacy of lightning rods is sometimes doubted, and an idea entertained that the rod often proves dangerous by attracting the stroke. It is difficult to say how many buildings have been saved by rods, as it is impossible to say what might or might not have taken place in their absence. It appears however that in Germany statistics have lately been furnished by insurance companies which support the opinion that rods offer a great degree of pro- tection. It has been found that the first point struck by lightning is that at which the great- est heating effect is produced, and that if no inflammable materials are present there, the danger of fire following the stroke is greatly diminished. Inflammable vapors form better conductors than the air, and if no rods are furnished to buildings when such vapors are issuing they are liable to be struck, and when struck more liable to take fire than if supplied with rods. The English association of tele- graph engineers have furnished still more valu- able information. The poles of their lines were frequently struck until they mounted them with wire running from the top to the ground. They have found that it is well to have a large mass of metal in the ground con- nected with the wire, and that the latter should be as straight as possible. The rod should be continuous, and present no points except at the top; insulation under such circumstances is not necessary. One of the most useful works for reference in regard to lightning and light- ning rods is the treatise of Sir W. Snow Harris " On the Nature of Thunder Storms, and on the Means of Protecting Buildings and Ship- ping against the Destructive Effects of Light- ning " (London, 1843).