Page:EB1911 - Volume 16.djvu/696

Rh from the latter to earth takes such an erratic course that according to the Lightning Research Committee “no series of lightning conductors of the hitherto recognized type suffice to protect the building.” In Germany two kinds of lightning stroke have been recognized, one as “zündenden” (causing fire), analogous to the B flash, the other as “kalten” (not causing fire), the ordinary A discharge. The destructive effect of the former was noticed in 1884 by A. Parnell, who quoted instances of damage due to mechanical force, which he stated in many cases took place in a more or less upward direction.

The object of erecting a number of pointed rods to form a lightning conductor is to produce a glow or brush discharge and thus neutralize or relieve the tension of the thunder-cloud. This, if the latter is of the A type, can be successfully accomplished, but sometimes the lightning flash takes place so suddenly that it cannot be prevented, however great the number of points provided, there being such a store of energy in the descending cloud that they are unable to ward off the shock. A B flash may ignore the points and strike some metal work in the vicinity; to avoid damage to the structure this must also be connected to the conductors. A single air terminal is of no more use than an inscribed sign-board; besides multiplying the number of points, numerous paths, as well as interconnexions between the conductors, must be arranged to lead the discharge to the earth. The system of pipes and gutters on a roof must be imitated; although a single rain-water pipe would be sufficient to deal with a summer shower, in practice pipes are used in sufficient number to carry off the greatest storm.

Protected Area.—According to Lodge “there is no space near a rod which can be definitely styled an area of protection, for it is possible to receive violent sparks and shocks from the conductor itself, not to speak of the innumerable secondary discharges that are liable to occur in the wake of the main flash.” The report of the Lightning Research Committee contains many examples of buildings struck in the so-called “protected area.”

Material for Conductors.—Franklin’s original rods (1752) were made of iron, and this metal is still employed throughout the continent of Europe and in the United States. British architects, who objected to the unsightliness of the rods, eventually specified copper tape, which is generally run round the sharp angles of a building in such a manner as to increase the chances of the lightning being diverted from the conductor. The popular idea is that to secure the greatest protection a rod of the largest area should be erected, whereas a single large conductor is far inferior to a number of smaller ones and copper as a material is not so suitable for the purpose as iron. A copper rod allows the discharge to pass too quickly and produces a violent shock, whereas iron offers more impedance and allows the flash to leak away by damping down the oscillations. Thus there is less chance of a side flash from an iron than from a copper conductor.

Causes of Failure.—A number of failures of conductors were noticed in the 1905 report of the Lightning Research Committee. One cause was the insufficient number of conductors and earth connexions; another was the absence of any system for connecting the metallic portion of the buildings to the conductors. In some cases the main stroke was received, but damage occurred by side-flash to isolated parts of the roof. There were several examples of large metallic surfaces being charged with electricity, the greater part of which was safely discharged, but enough followed unauthorized paths, such as a speaking-tube or electric bell wires, to cause damage. In one instance a flash struck the building at two points simultaneously; one portion followed the conductor, but the other went to earth jumping from a small finial to a greenhouse 30 ft. below.

Construction of Conductors.—The general conclusions of the Lightning Research Committee agree with the independent reports of similar investigators in Germany, Hungary and Holland. The following is a summary of the suggestions made:—

The conductors may be of copper, or of soft iron protected by galvanizing or coated with lead. A number of paths to earth must be provided; well-jointed rain-water pipes may be utilized.

Every chimney stack or other prominence should have an air terminal. Conductors should run in the most direct manner from air to earth, and be kept away from the walls by holdfasts (fig. 1), in the manner shown by A (fig. 2); the usual method is seen in B (fig. 2), where the tape follows the contour of the building and causes side flash. A building with a long roof should also be fitted with a horizontal conductor along the ridge, and to this aigrettes (fig. 3) should be attached; a simpler method is to support the cable by holdfasts armed with a spike (fig. 4). Joints must be held together mechanically as well as electrically, and should be protected from the action of the air. At Westminster Abbey the cables are spliced and inserted in a box which is filled with lead run in when molten.

Earth Connexion.—A copper plate not less than 3 sq. ft. in area may be used as an earth connexion if buried in permanently damp ground. Instead of a plate there are advantages in using the tubular earth shown in fig. 5. The cable packed in carbon descends to the bottom of the perforated tube which is driven into the ground, a connexion being made to the nearest rain-water pipe to secure the necessary moisture. No further attention is required. Plate earths should be tested every year. The number of earths depends on the area of the building, but at least two should be provided. Insulators on the conductor are of no advantage, and it is useless to gild or otherwise protect the points of the air-terminals. As heated air offers a good path for lightning (which is the reason why the kitchen-chimney is often selected by the discharge), a number of points should be fixed to high chimneys and there should be at least two conductors to earth. All roof metals, such as finials, flashings, rain-water gutters, ventilating pipes, cowls and stove pipes, should be connected to the system of conductors. The efficiency of the installation depends on the interconnexion of all metallic parts, also on the quality of the earth connexions. In the case of magazines used for explosives, it is questionable whether the usual plan of erecting rods at the sides of the buildings is efficient. The only way to ensure safety is to enclose the magazine in iron; the