Pittsburg dispatch. [volume] (Pittsburg [Pa.]) 1880-1923, 1891-01-18
BOLTS FROM THE SKY.
What Lightning Is and How to Pro.
tect Buildings From 1t.
ELECTRICITY IN THE CLOUDS
Is Generated BY Friction Between the
Yapor and the Air.
The MOST SCIENTIFIC FORM OF RoD
WRITTEN FOR THE prsrArcH.1

LII we rub hard piece of rubber briskly
with a dry silk handkerchief, electricity will
be generated on both the silk and the hard
rubber. These objects are then said to have
a different potential from that of the earth
it may be greater or it may be less. This
difference of potential means difference Of
pressure, and the tendency is to neutraliss
or become of the same pressure. For ex.
ample, stone on the top Of a cliff has q
difference of potential or pressure from .
stone at the bottom of the cliff. The ten
dency of the stone is to reach the bottom.
The cliff, with respect to the stone, would
correspond to the insulator, with respect to
electricity.
A reservoir of water has a potential or
pressure far greater than that of the ocean,
and so the water in the reservoir tends to
fall and seek its own level. The dam, hold.
ing the water back, again corresponds to an
electric insulator holding the electricity
back. If the dam IS too weak and gives
away, there will be rush of water. Air is
an electric dam or insulator. If two objects
having difference of electrical potential be-
tween them are brought near together, thers
will be tendency toward equilibrium Of
potential. If the difference OIL potential is
great enough to break down or through the
air-the electric dana discharge will
take place, indicated BY spark and snap.
The Beginning of Lightning.
VaDor, rising from the earth, generates
electricity hy friction, just as electricity was
generated by rubbing hard rubber with
silk handkerchief The vapor, as it rises,
Is blown about among the trees and hills
and the friction between the two generates
the electricity. Each drop OF vapor is
sphere with its own electric charge on the
surface; and as these drops join and form
larger ones, the electric charge ON each new
drop accumulates on the surface. Thus the
vapor accumulates all the time together with
the electric charge till finally we have ths
black thunder cloud.
Two such clouds formed over different
portions of the earth's surface are almost
sure to have a difference OF potential exist
lug between them, and if they are blown
toward each other q discharge will take
place from the cloud of higher potential to
that of the lower. A huge spark is seen and
we call it lightning. A great snap is heard
and we call it thunder. When the light
ming is hidden behind the clouds we call il
shut lightning. The same effect can be pro-
duced if bright electric spark is made be-
hind a screen in dark room.
Striking the Earth.
II the cloud has higher potential than
that of the earth discharge will take place
from the cloud to the earth-and similarly
if the earth has higher potential than the
cloud, a discharge Will take place from the
earth to the cloud. In every case, of course,
the difference of potential must be sufficient
to break down, or in other WORDS pass
through the insulation, which in this case is
the air. Observations show that
the distance through which the dis
charge takes place is anywhere from
few yards to several miles, according to
circumstances. When we compare the
length of the lightning spark with that of
the longest spark that can be generated by
our most powerful electric machines, it will
be easy to understand what an enormous
amount of energy is accumulated in the
clouds in the shape of electricity.
The zigzag path OF the lightning, although
imperfectly understood, is, no doubt, partly
due to different resistances offered to the
discharge bv different portions OF the atmos-
phere-electricity always taking the shortest
and easiest path. The rolling sound heard
after the first sharp report is supposed to be
due to the echo between clouds or between
the clouds and the earth. No doubt the
reader has noticed how that after a loud clap
of thunder large drops of rain are very apt to
fall. This is due to the concussion OF the air
small drops, which were before supported in
the cloud, are thus forced together and then.
being too heavy for their support, they fall.
The same reasoning explains the shower that
is almost sure to follow battle, when there
is great deal of cannonading.
Lightning Loves Point.
I have stated that lightning takes the
shortest and easiest path. The nearest point
is therefore in the shortest path, hence
trees, church steeples and house tops are apt
to be struck. 1t has also been Iound that
the lightning discharge will take place to or
from point more'readily than to or from a
fiat surface. 1t is for these two reasons,
therefore, that lightning rods, as we see
them to-day, form protection lor buildings
against damage rom lightning discharges.
The rules, however, for the construction and
erection of lightning rods are not often un-
derstood, or, ii so, they are III many cases
grossly neglected.
The following are the most important
points that should be observed: The space
covered by any one lightning rod should not
be greater than the cone, whose height is the
height of the rod and whose base has a di-
ameter OF twice this distance. Points on the
lightning rod, as above noticed, are an ad.
vantage. These should, however, not be too
sharp, otherwise, having little ruass Of
metal, they would readily fuse at the first
discharge. Three slightly blunted points
are better than one sharp one. 1t is well to
make those points OF platinum, or, if made
OF copper, they should be plated to prevents
oxidation.
Glass Insulators Not Good.
The rod itself should be firmly connected
to the building with metal supports. A
great mistake is often made here in the usa
of glass insulators. AS the lightning must
reach the ground, the rod should have the
best possible ground connection, and as a
house with good foundation is practically
a part of the ground, it ollows that good
metal supports are an advantage. OF course.
wooden houses without foundation should
not be thus connected. All joints should be
well soldered.
The best material for a lightning rod is
copper. Its weight should be about seven
ounces per foot If iron is used the
weight should be about two and one-haR
pounds per loot. Iron rods should be painted
to protect them from rust The form of the
rod is not important. Copper tape, how
ever, has the advantage of easy erection
without joints which are always bad; but
with tape there Is a great temptation to make
sharp bends in the rod to conform closely
with the outlines of the building. This is q
mistake, for recent observations show that ,
sharp bends offer great resistance to the pas
sage of the lightning discharge The dis
tance between any two points around bend
should not be any greater than twice the dis
tance between them In straight line. The
rod should never be connected in any way
with the gas pipes of the house, as poor
points are apt to cause spark, which would
ignite the gas and perhaps c.. use an explo-
sion. 1t is we'll to connect to iron water