Page:The New International Encyclopædia 1st ed. v. 12.djvu/291

the water in the ciucks, aud thus breaks up the chimney by the explosion, when the discharge runs along the roots of a tree or an underground drain it tears up the ground by the force of the explosive expansion of the water.

In general, perfect protection from a destruc- tive Hash can only be obtained by surrounding an object with a continuous metallic covering. This need not necessarily be a metal bo.x, but may consist of a network of wire, or even a few metallic rods properly arranged; thus for an ordinary building the rods should run vertically up and down each of the corners and horizontally along tlie ridge, the eaves, and every other e<lge or prominence on the roof; these are the ex- posed portions, and if they are well protected there is but slight chance that a flash will strike the building elsewhere. A partial protection is of course provided for by a single metallic wire or so-called lightning-rod, especially if the upper end protrudes far aljove the building, but it is quite deceptive to suppose that any definite rule can be ollered for calculating the so-called area of protection.

The conductors usually found on buildings have as their terminals sharp cones or prongs of copper which are gilded to protect the points and to prevent corrosion, while the conductors them- selves are generally in the form of a metal rod, rope, or ribbon, and terminate either in plates buried in damp ground or by making connection with water or gas pipes. Modern authorities agree that the more points along the ridges and eaves of a roof, the greater the amount of safety, and these of course should point to the sky. A fac- tory chinmey or steeple in the neighborhood is apt to attract the lightning and in all cases should be protected by conductors, as it may be a source of danger to surrounding buildings and not a safeguard as is commonly supposed. The lightning-conductor should be of iron or copper, and if the latter material is employed, it is con- sidered advantageous to use it in the form of tape weighing about six ounces to the foot. Iron, however, answers as well, being preferred by some authorities, and can be used in the shape of rod or tape with a weight of about thirty-five ovuices to the foot. The lightning-conductor should be of the same material and continuous throughout, joints being either avoided or constructed most carefully. In its course to the earth care should be taken not to have sharp bends, corners, or curves. An imjiortant consideration is the con- nection with the ground, and while it is possible to connect directly with water-pipes, an inde- pendent ground is far better; running water, a deep well or boring, can be utilized with great advantage as a terminal for the eonductor. . ton of charcoal or coke buried in the ground and a trench filled with the same material and leading to water also make a good ground. In the United States there are few lightning flashes so intense that a good lightning-rod, well earthed, will not provide an easy path for the lightning; and while such protection is not neces- sary for houses in the city built in close blocks with metallic roofs and cornices, yet for bams and dwellings in the country authorities unite in pronouncing good lightning-conductors eminently desirable.

The Washington Monument at Washington, D. C, has furnished one of the best instances of the eft'ectivencss of lightning-conductors as a protective device. As it stands 555 feet in height in the centre of flat and well-watered ground, it should be a conspicuous mark for lightning. It is protected by means of four cop- per rods attached to the aiuniinum cap of the pyraniidion at the top, and connecting with the uprights supporting the elevator machinery. These copper rods are connected to each other by additional copper rods, and every five feet along the system there are copper points, num- bering in all about 200. In the fifteen years elapsing since this improved arrangement was introduced, the monument has not been injured in any way by lightning. Previously, when the aluminum point was less effectively connected with the vertical supports, there was one light- ning stroke, which, while causing hut slight dam- age to the cap, nevertheless occasioned the pro- vision of additional conductors. Dr. Oliver Lod<»e. the celebrated English authority on the subject of lightning-conductors, who h.is within a few- years advanced some novel ideas on this matter which have tended to change previous theories, says, "Almost any conductor is probably better than none, but few or no conductors are abso- lute and complete safeguards." For further in- formation, consult: Lodge, lAiihlning Conductors and Li'ihtning (ruards (London, 1892); and I'nited States Weather Bureau bulletins on lightning.

LIGHTNING-ARRESTERS. Devices tn ])rotect electrical apparalus from damage by lightning. In the early days of telegraphy and telephony many accidents were caused by light- ning striking the wires and running into the offices, often injuring the operator and destroy- ing property. In order to obviate these dilTieul- ties an instrument was devised and called a 'lightning-arrester.' The original simple form is shovn in Fig. I. The three plates A, B, C

Fig. 1.

are mounted upon an insulating block. For example, the plate A is connected to the ground, B to the telegraph or telephone instrument, and C to the line wire, and to the other side of the instrument. In ordinary operation the currents from the line cannot go from C across to A with- out going through the instrument, but if the line is struck by lightning the latter conies to C, and by reason of its high pressure jumps from the jagged edges of C over to A and B. without going through the other instniments at all. on account of their high impedance (effective resistance). Thus the so-called arrester provides a cross-cut, or short-circuit, so that the lightning reaches the earth without going through the telegraph or