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LIGHTNING

1069

LIGHTNING RODS

in reference to lighthouses was passed in 1789, and there are about 3,000 lights and buoys on the seacoast and lakecoasts.

Most of the earlier lighthouses were constructed as aids to coasting on inclosed waters and straits rather than as aids to open-sea navigation; and therefore great height of tower was seldom required. Elevat.d spots were of course selected for seacoast lights; but even in the early history c 1 navigation it was seen that there were many dangers to ships and seamen that could not be avoided by lighthouses constructed on land. It therefore became a problem of engineering to build them upon solid rock a greater or lesser distance from the mainland, and so that they must be able to withstand not only the heaviest winds and storms but the tides and waves of the sea. The most noted case is that of the Eddystone lighthouse (which see). The use of lamps led to the invention of reflectors, the early ones being about 18 inches in diameter. It was reserved for Argand to devise the cylindrical wick-burner, and in 1822 Augustin Fresnel contrived the dioptric system for lighthouse purposes. Electric lights have been used to some extent; but there are some disadvantages connected with the generation and use of electricity, which have thus far prevented its general adoption. A light at an elevation of 40 feet above the sea — its power and intensity supposed to be adequate — will have a range of about 7 miles; at 100 feet, a range of ii miles; at 200 feet, a range of 16 miles; and, as the mariner's eye is generally supposed to be about 15 feet above the water, to each of these estimates must be added the range for that elevation, four miles and a fraction. See D. P. Heap's Ancient and Modern Lighthouses.

Lightening, an electrical phenomenon taking place in the earth's atmosphere. Immediately after the discovery of the electrical machine a similarity between its discharge and the lightning discharge was noticed. But it was not until Benjamin Franklin obtained a discharge from clouds by means of a kite that the connection between electricity and lightning was thoroughly established. It is evident that lightning consists in a discharge sometimes from the cloud to the earth and sometimes from one cloud to another. But what causes these charges, how the clouds become electrified, has not yet been explained.

At various meteorological stations over the surface of the earth the electrical potential of the earth's atmosphere is daily measured. But the electrical pressures thus determined are infinitesimal when compared with those which produce lightning; and the connection of the differences of potential in the thin layer of air, which can thus be measured, with the weather is very imperfectly understood. It is well-known

that in the ordinary Leyden jar discharge the electrification surges to and fro several times before the discharge ceases. This is called an oscillatory discharge. There is some evidence, obtained from photographs of lightning flashes, for thinking that the discharge from cloud to earth frequently is oscillatory. See Lodge's Lightning ana Lightning (Conductors.

Lightning Arrest'er, a device for protecting electrical apparatus from lightning or atmospheric electricity. They are desirable whenever any part of an electrical circuit is outdoors. In some places, as in England, lightning mishaps are rare, while in other places, as in the Rocky Mountains, even with the best protectors it often is necessary during storms to shut down an electric plant on account of the danger from lightning. The lightning-discharges through the machines so puncture the insulation that the machinery is ruined. The ordinary lightning arrester, such as has been used on telegraph lines for many years, consists of an air gap with sharp points, SQr that the lightning discharge will jump across this gap and thus reach the ground sooner than pass through the apparatus. Practically all modern lightning arresters are on this principle, but for large dynamo circuits it is necessary to add some device for breaking an electric arc which will be formed across the air gap. In the Thomson lightning arrester this is done by having the poles of an electro-magnet opposite the air gap. This "blows out" the arc. In the Wurtz arrester the discharge is between a series cf rods of non-arcing metals. A line may often be protected by stretching above it an ordinary barbed wire which is connected with the ground every few feet.

Lightning Rods, a device for protecting buildings and ships from the destructive effects of lightning. It has been proved that the most effective lightning rods are made by using flat strips of metal (practically either iron or copper) which at their upper ends are pointed, at their lower ends are connected with the earth in a thorough manner, and at intermediate points ara connected to any portions of the building which are good conductors, as a metal roof or water pipes. The lightning rod in this form was suggested by Benjamin Franklin. Contrary to the popular idea, its chief function is not to protect the house which lie* directly in the path of the discharge, but rather to prevent discharges by leading the induced charges off gently. (See ELEO TRICITY.) Discharges as rapid as those which occur in lightning confine themselves exclusively to the outside of the conductor. It is essential, therefore, that the conductor present a large surface in order to carry off a large discharge rapidly. Glass insulators for lightning rods are no value, and may be of positive harm. Too much importance can-