<h2><SPAN name="CHAPTER_VIII" id="CHAPTER_VIII"></SPAN>CHAPTER VIII.</h2>
<h3>ATMOSPHERIC ELECTRICITY.</h3>
<p>Nature has another mode of generating electricity,
called atmospheric. The normal conditions
of potential between the earth and the
upper atmosphere seem to be that the atmosphere
is positively electrified and the earth
negatively. These conditions change, apparently
from local causes, for short periods
during storms. In some way the sun's rays
have the power directly or indirectly to give
the globules of moisture in the air a potential
different from that of the earth.</p>
<p>In clear weather we find the air near to the
earth in a neutral condition, but gradually assuming
the condition of a positive charge as we
ascend; so that the upper air and the earth are
oppositely charged like the two sides of a Leyden
jar or two leaves of a condenser. This
condition is intensified and localized when a
thunder-cloud passes over the earth. The
moisture globules have been charged with potential
energy by the power of the sun's rays
when evaporation took place; but in this state
the energy is neither heat nor electricity, but<span class="pagenum"><SPAN name="Page_78" id="Page_78"></SPAN></span>
a state of strain like a bent bow or a wound-up
spring. When these moisture globules condense
into drops of water the potential energy
is set free and becomes active either as heat or
electricity. The cloud gathers up the energy
into a condensed form, and when the tension
gets too great a discharge takes place between
the cloud and the earth or from one cloud to
another, which to an extent equalizes the
energy.</p>
<p>In most cases of thunder and lightning it
is only a discharge from cloud to cloud unequally
charged. This does not relieve the
tension between the earth and the cloud, but
distributes it over a larger area. The reason
for this constant electrical difference between
the earth and the upper regions of atmosphere
is not well understood, except that primarily it
is an effect of the sun's rays. Evaporation
may and probably does play a part, and the
same causes that give rise to the auroral display
may contribute in some way to the same
result. Evaporation does not always take
place at the earth's surface. Cloud formations
may be evaporated in the upper air into invisible
moisture spherules, and charged at the
time with potential energy. If we go up into
a high mountain when the conditions are right,
we can witness the effect of this condition of
electrical charge or strain between the upper
regions of the atmosphere and the earth, and<span class="pagenum"><SPAN name="Page_79" id="Page_79"></SPAN></span>
the tendency to equalize the potentials between
the clouds and the earth. Often one's hair
will stand on end, not from fright, but from
electricity passing down from the upper regions
to the earth. When the tension is very
great a loud hissing sound as of many musical
tones of not very good quality may be heard,
and a brush or fine-pointed radiation of electricity
may be seen from every point, even
from your finger-ends. The thunder is not
usually so loud on high mountains for two
reasons—one because the air is rare, but the
chief reason is that the mountain acts as a
great lightning-rod and gradually discharges
the cloud or atmosphere, for often the phenomena
may occur when the sky is clear.</p>
<p>I remember being on top of what is called
the Mosquito Range, between Alma and Leadville
in Colorado, during the passage of a
thunder shower. There was no heavy thunder,
but a constant fusillade of snapping sounds, accompanied
by flashes not very intense. I could
feel the shocks, but not painfully. A part of
the time I was in the cloud and became for the
time being a veritable lightning-rod. After
the cloud passed it crawled down the mountainside
as if clinging to it, all the time bombarding
it with little electric missiles. After the
cloud left the mountain and passed over
the valley I could hear loud thunder, because
the charge would have to accumulate quite a<span class="pagenum"><SPAN name="Page_80" id="Page_80"></SPAN></span>
quantity, so to speak, before it could discharge.
These heavy discharges when the cloud is some
distance from the earth would be dangerous to
life, while the light ones, when the cloud is
in contact with the earth, are not.</p>
<p>Many wonderful and destructive effects
come from these lightning discharges and
many lives are lost every year from this cause.
I do not suppose it is possible to be on one's
guard continually, but many lives are needlessly
lost either from ignorance or carelessness.
Although there is a just prejudice
against lightning-rods as ordinarily constructed,
it is still just as possible to protect
your house and its inmates from the destroying
effects of lightning as from rain. If, for
instance, we lived in metal houses that had
perfect contact all round them with moist
earth, or better, with a water-pipe that has a
large surface contact with the earth, the lightning
would never hurt the house or the inmates.
In such a case you simply carry the
surface of the earth to the top of your house,
electrically speaking, and neutralization takes
place there in case the lightning strikes the
house. A house that is heated with hot water
can easily be made lightning-proof by a little
work at the top and bottom of the heating system.
All the heavy metal of the house should
be a part of the lightning-rod. Points should
be erected at the chimneys, and if there is a<span class="pagenum"><SPAN name="Page_81" id="Page_81"></SPAN></span>
metal roof they should be connected with it.
Then connect the roof with rods from several
points with the ground. Here is where most
rods fail. The ground connection is not
sufficient. The earth is a poor conductor, and
we have to make up by having a large metal
surface in contact with it. It is best to have
the rod connected with the water pipe, if there
is one, and have it connected with metal running
all around the house as low down as the
bottom of the cellar, for sometimes there is an
upward stroke, and you never can tell where it
is coming up. If you have a heating system
it should be thoroughly grounded and the top
pipe connected with the rods at the chimneys.
These rods need not be insulated as is the
usual practice.</p>
<p>If you are outdoors during a thunder-storm
never get under a tree, but if you are twenty
or thirty feet away it may save your life, because,
if it comes near enough to strike you,
it will probably take the tree in preference.
It seeks the earth by the easiest passage. An
oil-tank and a barn are dangerous places, if
the one has oil in it and the other is filled with
hay and grain. A column of gas is rising that
acts as a conductor for lightning. Of course
if the barn is properly protected with rods it
will be safe. Sometimes a cloud is so heavily
charged that the lightning comes down like an
avalanche, and in such a case the rods must<span class="pagenum"><SPAN name="Page_82" id="Page_82"></SPAN></span>
have great capacity and be close together to
fully protect a building.</p>
<p>There is a popular notion that rods draw the
lightning and increase the damage rather than
otherwise. This is a mistake. Points will
draw off electricity from a charged body
silently. It would be possible to so protect a
district of any size in such a way that thunder
would never be heard within its boundaries if
we should erect rods enough and run them
high enough into the upper air. The points—if
they were close enough together—would
silently draw off the electricity from a cloud
as fast as it formed, and thus effectually prevent
any disruptive discharge from taking
place.</p>
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