The aegis & intelligencer. [volume] (Bel Air, Md.) 1864-1923, 1906-07-27
SAFETY FROM lightning.

The United States Department of Agri-
culture has issued booklet on lightning
conductors. It's particularly interesting,
Il is said that of late years, the use of
| lightning conductors has fallen off in large
cities. This is owing to the prevalence
buildings with metal roofs.
For buildings that stand isolated in Il he
open country, the prudent course would
be to install thereon a system of protec-
ton from lightning The booklet treats
of the nature of protection each building
should have, and gives as safe rule that
some dcfinite relation to the value of the
building. In arriving at decision, it is
asserted that while absolute security from
damage by lightning is attainable only
with great difficulty and considerable ex-
pense, a reasonable degree of protection
can be secured by very simple means,
provided the work is done by thoroughly
competent persons. The book gives the
following interesting features in the re-
port of the Lightning Research Com
mittee.
at has been pointed out by Sir Oliver
Lodge that lightning discharges are of
two distinct characters, which he has
named the A flash and the flash, re-
spectively. The A fl lsh is of the simple
type which arises when an electrically
oil, Cln hac of
the earth without an intermediate cloud
intervening, and under these conditions
the ordinary type of lightning conductor
acts in two ways: First, by silent dis
charge, and, secondly, by absorbing the
energy of descriptive discharge. In the
second type, B, where another cloud in
tervenes 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 sud
denly relieved, and the descriptive dis
charge from the latter to the earth takes
such an erratic course that no series of
lightning conductors of the hitherto
recognized type suffice to protect the
building.
mo cases of damage to modern steel
frame structures have come under the
notice of the committee. The ordinary
method of construction, however, in this
country does not provide full protection.
In many cases the steel columns stand on
stone foundations and the metal is not
carried deepenough for effective earthing.
The metal columns ought to be earthed
at the time of construction"
The following practical suggestions are
made.
Two main lightning rods. one on each
side, should be provided, extending from
the top of each tower, spire. or high
chimney stack by the most direct course
to earth.
Horizontal conductors should connect
all the vertical rods (a) along the ridge,
or any other suitable position on the
roof; (h) at or near the ground.
The upper horizontal conductor should
be fitted with aigrettes or points at i'll
tervals of 20 or 80 feet.
Short vertical rods should be erected
along minor pinnacles and connected
with the upper horizontal conductor.
All roof materials, such as finals, ridg-
ing, rain water and ventilating pipes,
metal cowls, lead flashing, cutters, etc.
should be conducted to the horizontal
conductors.
All large masses of metal in the build
ing should be connected to earth either
directly or by means of the lower hori-
Zoutal conductor.
Where roofs are partially or wholly
metal lined they should be connected to
earth by means of vertical rods at several
points.