THUNDER-ROD, is a rod of metal at tached generally to a side of a building, and extending from below the level of the ground to a point several feet above the highest part of the roof of the building, in order to secure the edifice from the effects of thunder or lightning : the upper extremity of the rod or bar terminat ing in a point. It is to Dr. Franklin that the world is indebted for the idea of raising pointed rods in order to secure buildings from the effects of atmospherical electricity; and the recom . mendation was immediately adopted both for edifices on land and ships on the water.
The thunder-rod should be thick enough to carry the electric fluid to the ground without being melted by it ; in general a cylindrical rod about half an inch in diameter will be sufficient to prevent this effect from taking place ; whether of iron or copper, it should be covered above ground with a coating of paint ; and the part under ground is usually formed with two or more branches in order to facilitate the passage of the electric fluid to the earth.
A ship at sea, like an edifice on land, may, when there is an accumulation of electric matter in the upper part of the atmosphere, be struck aloft; or, when the atmosphere is in a contrary state, the lower part of the ship may be struck, the lightning in the latter case ascending along the mast: and ships un furnished with metallic conductors have fre quently suffered serious injury during thunder storms, while those which have been so pro vided have generally escaped. The rigid bars
of Franklin are considered inapplicable, as conductors, to ships, and instead of them chains of copper have been generally employed ; these are attached to the masts at their upper extremities; and, following the standing rig ging, they pass down the ship's sides into the water. In 1822 Mr. (now Sir W.) Snow Harris proposed, and subsequently cruised to be executed for ships, conductors consisting of slips of copper, of sufficient thickness to prevent them from being fused ; these slips are inserted, in two layers, in a groove cut longitudinally along the mast, the joints of one layer being opposite to the middle parts of the other, and they are fastened to the mast by copper screws. The whole line of metal passes down from the copper spindle at the top of the mast-head, and at the junc tions of the upper and lower masts the ship is made to join a cylinder of copper which lines each sheave-hole; the lower part of the line is connected with a plate of copper which is fixed on the keelson, at the step, and from thence there is a communication with the water by three copper bolts which pass quite through the keel. This improvement by Sir W. Snow Harris, has proved to be a most valuable one ; the safety thereby ensured to the royal shipping, (so far as the effects of lightning are concerned), represents, iu money value, a sum which must be immense, however difficult to estimate.