[1] _Journ. Inst. Elec. Eng._ 28, p. 1. The authors of this paper
give numerous instructive curves taken with the oscillograph, showing
the form of the arc P.D. and current curves for a great variety of
alternating-current arcs.
LIGHTNING, the visible flash that accompanies an electric discharge in
the sky. In certain electrical conditions of the atmosphere a cloud
becomes highly charged by the coalescence of drops of vapour. A large
drop formed by the fusion of many smaller ones contains the same amount
of electricity upon a smaller superficial area, and the electric
potential of each drop, and of the whole cloud, rises. When the cloud
passes near another cloud stratum or near a hilltop, tower or tree, a
discharge takes place from the cloud in the form of lightning. The
discharge sometimes takes place from the earth to the cloud, or from a
lower to a higher stratum, and sometimes from conductors silently. Rain
discharges the electricity quietly to earth, and lightning frequently
ceases with rain (see ATMOSPHERIC ELECTRICITY).
LIGHTNING CONDUCTOR, or LIGHTNING ROD (Franklin), the name usually given
to apparatus designed to protect buildings or ships from the destructive
effects of lightning (Fr. _paratonnerre_, Ger. _Blitzableiter_). The
upper regions 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
discharges were distinguished by Sir Oliver Lodge into two distinct
types--the _A_ and the _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 carrying 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 "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.
[Illustration: FIG. 1.--Holdfast.]
[Illustration: FIG. 2.]
[Illustration: FIG. 3.--Aigrette.]
[Illustration: FIG. 4.--Holdfast on Roof.]
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.
[Illustration: _Fig_. 5.--Tubular Earth.]
_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 next best is to arrange
the conductors so that they surround it like a bird cage.
BIBLIOGRAPHY.--The literature, although extensive, contains so many
descriptions of ludicrous devices, that the student, after reading
Benjamin Franklin's _Experiments and Observations on Electricity made
at Philadelphia_ (1769), may turn to the _Report_ of the Lightning Rod
Conference of December 1881. In the latter work there are abstracts of
many valuable papers, especially the reports made to the French
Academy, among others by Coulomb, Laplace, Gay-Lussac, Fresnel,
Regnault, &c. In 1876 J. Clerk Maxwell read a paper before the British
Association in which he brought forward the idea (based on Faraday's
experiments) of protecting a building from the effects of lightning by
surrounding it with a sort of cage of rods or stout wire. It was not,
however, until the Bath meeting of the British Association in 1888
that the subject was fully discussed by the physical and engineering
sections. Sir Oliver Lodge showed the futility of single conductors,
and advised the interconnexion of all the metal work on a building to
a number of conductors buried in the earth. The action of lightning
flashes was also demonstrated by him in lectures delivered before the
Society of Arts (1888). The Clerk Maxwell system was adopted to a
large extent in Germany, and in July 1901 a sub-committee of the
Berlin Electro-technical Association was formed, which published
rules. In 1900 a paper entitled "The Protection of Public Buildings
from Lightning," by Killingworth Hedges, led to the formation, by the
Royal Institute of British Architects and the Surveyors' Institution,
of the Lightning Research Committee, on which the Royal Society and
the Meteorological Society were represented. The _Report_, edited by
Sir Oliver Lodge, Sir John Gavey and Killingworth Hedges (Hon. Sec.),
was published in April 1905. An illustrated supplement, compiled by K.
Hedges and entitled _Modern Lightning Conductors_ (1905), contains
particulars of the independent reports of the German committee, the
Dutch Academy of Science, and the Royal Joseph university, Budapest. A
description is also given of the author's modified Clerk Maxwell
system, in which the metal work of the roofs of a building form the
upper part, the rain-water pipes taking the place of the usual
lightning-rods. See also Sir Oliver Lodge, _Lightning Conductors_
(London, 1902). (K. H.)