The part of the lightning-conductor forming the connection between the rod and the ground is generally a prismatic or cylindrical rod of iron (the latter being preferable), or a strap of copper; sometimes a rope of iron or copper wire is used. Iron wire improves as a conductor when electric currents pass through it; copper wire, in the same circumstances, becomes brittle. An iron rope is much better, therefore, for con ducting than a copper one. Galvanized iron is, of all materials, the best for conductors. 'The conducting-rod ought to be properly connected with the conical rod either by rivet in,g or soldering or both. Here, as at overy point of juncture, the utmost care must be taken that there is no break in the conduction. The conducting-rod is led along the Tool and down the outside of the walls, and is kept in its position by holdfasts fixed in the building. There must be no sharp turns in it, but each bend must be made as round as possible. Considerable discussion has arisen as to the proper thickness for the -conducting-rod, If it were too small it would only conduct part of the electricity, and leave the bitilding to conduct the rest, and it tnight be melted by the electricity endeavor ing to force a passage through it as an insufficient conductor. The Paris commission, which sat in 1823. gave the minimum section of an iron conductor as a square of 15 millimeters (about three-fifths of an inch) in side, and this they considered quite suffi cient in all circumstances. A rod of copper would need to be only two-fifths of this, aS copper conducts electricity about six times more readily than iron, This calculation is very generally followed in practice. In leading the conductor along the building it .should be kept as much apart as possible from masses of conducting matter about the building, such as iron beams, machinery, etc. These may form a broken chain of con ductors communicating with the ground, and divert a portion of the electricity from. the lig-htning-condnetor. If such took place, then at each interruption electricity would pass in a visible and dangerous way-, and the efficacy of the conductor Nvould be lost. If the conductor cannot be properly insulated from these masses of metal, the necessary security is got by putting thein in connection with the conductor, so as to forrn a part of it. Water-runs, leaden roofs, and the like, must, for this reason, all be placed in conducting connection with the conductor.
The portion of the lightning-conductor which is placed in the ground is no less worthy of attention than the other two. Should the lower part of the conductor end in ,dry earth, it is worse than useless, for when the lightning, attracted by the prominence and point of the upper rod, strikes it, it finds, in all likelihood, no passage through the uncouducting dry earth, and, in consequence, strikes off to a part of the ground where it may easily dispere itself and be lost. Wherever it is practicable, a lightning-con ductor should end in a well or large body of water. Water is a good conductor, and having various rainifications in the .soil, offers the best facility to the electricity to become dispersed and harmless in the ground. The rod on reaching the ground should be let down a foot and a half, or 2 ft., into the soil, and then turned away at right angles to the wall from the building in a, horizontal drain filled with charcoal, for about from 12 to 16 ft., and then turned into the well so far that its termination is little likely to be left dry. Where a well cannot be made, a hole 6 in. wide (wider, if possible) should be bored, from 9 to 16 ft., the rod placed in the middle of it, and the intervening space closely packed with freshly heated charcoal. The charcoal serves the double pur pose of keeping the iron.from rusting, and of leading away the electricity from the rod into the ground.
Lightning-conductors, when constructed with care, have been proved beyond a doubt to be a sufficient protection from the ravages of lightning. The circle within which a lightning-conductor is found to be efficacious is very limited. Its radius is generally Assumed to be twice the height of the rod. On large buildings, it is therefore necessary to have several rods, one on each prominent part of the building, all being connected so as to form one conducting system. In ships, a rod is placed on every mast, and their •connection with the sea is established by strips of copper inlaid in the masts, and attached below to the metal of or about the keel.
are appearances soinetimes found on the skin or clothing of men or animals that are either struck by lightning. or are in the vicinity of the stroke, and currently believed to be photographic representations of surrounding objccts or scenery. The existence of such prints appears, from. a theoretical point of view, highly improbable, as the essential conditions of forming a photographic image are wantino.; -still, several apparently well-authenticated instance,: have been recorded, which have le% -scientific authorities to give at least partial credence to them. One or two instances may serve to give a general idea of what are meant by lightning-prints. At Candelaria