LIGHTNING CONDUCTOR, or, according to Benjamin Franklin, "lightning rod," the name usually given to apparatus designed to protect buildings or ships from the destructive effects of lightning (Fr. paratonnerre, Ger. Blitzableiter). The upper re gions of the atmosphere being at a different electrical potential from the earth, the thick dense clouds which are the usual prelude to a thunder storm serve to conduct the electricity of the upper air down towards the earth, and an electrical discharge takes place across the air space when the pressure is sufficient. Lightning dis charges were distinguished by Sir Oliver Lodge into two distinct types, the A and B flashes. The A flash is of the simple type which arises when an electrically charged cloud approaches the earth without an intermediate cloud intervening. In the second type B, where another cloud intervenes between the cloud carry ing the primary charge and the earth, the two clouds practically form a condenser and when a discharge from the first takes place into the second the free charge on the earth side of the lower cloud is suddenly relieved, and the disruptive discharge 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 ziindenden (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 light ning 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 can not 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 dam age to the structure this must also be connected to the conductors. A single air terminal is insufficient. Besides multiplying the number of points, numerous paths, as well as interconnections between the conductors, must be arranged to lead the discharge to the earth. 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 )f 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." Franklin's original rods (1752) were made of iron, and this metal is still employed throughout the continent of Europe and in he United States. British architects, who objected to the unsight liness of the rods, eventually spe cified copper tape, which is gen erally 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 pro tection a rod of the largest area should be erected, whereas a single large conductor is far in ferior to a number of smaller ones and copper as a material is not so suitable for the purpose as iron, which offers more impe dance, and allows the flash to leak away by damping the oscillations. Iron is not suitable in cities, or factories where coal is used.
In 1876 J. Clerk Maxwell read a paper before the British Asso ciation in which he brought for ward the idea (based on Fara day's experiments) of protecting a building from the effects of lightning by surrounding it with a sort of cage of rods or stout wire.