ATMOSPHERIC ELECTRICITY.
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.
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 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.
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 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.
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 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.
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 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.
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 have great capacity and be close together to fully protect a
building.
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.