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.