The morning news. [volume] (Savannah, Ga.) 1887-1900, 1891-01-28
BOLTS FROM THE SKY.
What Lightning is and How TO Pro-
tect Buildings From 1t.
From. the Pittsburg Dispatch.


tect Buildings From 1t.
From the Pittsbur0 Dispatch.
If 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
different potential from that of the earth
it may be greater or 1t may be less. This
difference of potential means a differ ence of
pressure, and the tendency is to neutralize
or become Of the same pressure. For ex-
ample, a stone on the top of a cliff has a
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
iBg 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 ia
an electric dam or insulator. If two objects
having difference OF electrical potential
between them are brought near together,
there will be tendency toward equilibrium
of potential If the difference of potential
is great enough to break down or through
the dam-the electric dana discharge
will take place, indicated by spark and q
snap.
Vapor, rising from the earth, generates
electricity by friction, just as electricity
was generated by rubbing hard rubber with
a silk handkerchief. The vapor, as it rises,
is blown abyut among the trees and hills,
and the friction between the two generates
tho electricity. Each drop Of vapor is a
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
the black thunder cloud.
Two such clouds formed over different
portions of the earth'S surface are almost
sure to have difference of potential exist
ing between them, and if they are blown
toward each other a 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
lightning is hidden behind the clouds we
call it shut lightning. The same effect can
be produced if bright electric spark is
made behind q screen in dark room.
IL 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, 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 discharge
takes place is anywhere from a few yards
to several miles, according TO circum-
stances. 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 Ngzag path of the lightning. although
imperfectly understood, is, no doubt partly
due to the different resistances offered TO
the discharge by different portions of the
atmosphere 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 loud clap of thunder large drops Of
rain arc very apt to fall Tn is due to the
conclusion of the air-small drops, which
were before supported in the cloud, are
thus forced together and then, being too
u0dNJ bu6J 1dIls no
same reasoning explains the shower that is
almost sure to follow a battle, when there
is great deal of caunonading.
/ 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. It has also been found that
the lightning discharge will take place to or
from point more readily than to or from
fat 10 fhpqa
therefore. that lightning rods, as we see
them to-day, form protection OF buildings
against damage from lightning discharges.
The rules, however, for the construction
and erection of lightning rods are not often
understood, or, if 80, they are In 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 tina the cone, whose hight is the
hight of the rod, and whose base has a
diameter of twice this distance. Points on
the lightning rod, as above noticed, are an
advantage. These should, however, not be
too sharp, otherwise. having little mass 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 prevent
oxidation.
The rod itself should be firmly connected
to the building with metal supports. A
great mistake is often made here in the use
of glass insulators. As the lighting must
reach the ground, the rod should have the
best possible ground connection, and as
house with good foundation is practically
a part of the ground, it follows that good
metal supports are an advantage. OF course,
wooded houses without foundation should
not be thus connected. All joints should be
well soldered.
The best material for lightning-rod is
copper. Its weight should be about seven
ounces per foot. IL iron is used the weight
should be about two and one-half pounds
per foot. Iron rods should be painted to
protect them from rust. The form of the
rod is not important. Copper tape, however,
has the advantage Of easy erection without
joints, which are always bad; but with tape
fhara Is a grant farnntation TO make Sham