By W. SLINGO.
Lately we have all felt, I doubt not, a considerable amount of interest in
the various phenomena attending this summer's unusually heavy
thunderstorms, accompanied, as they have been, by vivid lightning
discharges of a more or less hurtful nature. The list of disasters
published in _Knowledge_, No. 143, might be very materially augmented were
we to record such damage as has been wrought since that list was compiled.
There is not, I suppose, in the mind of any intelligent man at the present
day a doubt as to the electrical origin of a lightning flash. The
questions to be considered are rather whence comes the electricity, and in
what way is the thunderstorm brought about. In attempting to answer these
questions, sight must not be lost of the fact that the very nature of
electricity is in itself almost sufficient to baffle any effort put forth
to ascertain from lightning, as such, its whence and its whither.
It is possible, however, with the aid of our knowledge of static
electricity, to arrive at hypotheses of a more than chimerical nature. In
the first place, that our sphere is a more or less electrified body is
generally admitted. More than this, it is demonstrated that the different
parts of the earth's surface and its enveloping atmosphere are variously
charged. As a consequence of these varying charges, there is a constant
series of currents flowing through the various parts of the earth, which
show themselves in such telegraph wires as may lie in the direction
followed by the currents. Such currents are known as earth currents, and
present phenomena of a highly interesting nature. But, apart from these
electrical manifestations, there is generally a difference of electrical
condition between the various parts of the earth's surface and those
portions of the atmosphere adjacent to or above them. Inasmuch as air is
one of the very best insulators, this difference of condition (or
potential) in any particular region is in most cases incapable of being
neutralized or equilibrated by an electric flow. Consequently the air
remains more or less continually charged. With these points admitted as
facts, the question arises, Whence this electricity? There have been very
many and various opinions expressed as to the cause of terrestrial
electricity, but far the greater portion of such theories lack fundamental
probability, and indicate causes which cannot be regarded as sufficiently
extensive or operative to produce such tremendous effects as are
occasionally witnessed. I take it that we may safely regard the evolution
of electricity as one of the ways in which force exhibits itself, that, in
other words, when work is performed electricity may result. When two
bodies are rubbed together, electricity is produced, so also is it when
two connected metals are immersed in water and one of them is dissolved,
or when one of the junctions of two metals is raised to a higher
temperature than the other junction. I will go further than this, so far,
in fact, as to maintain that there is a reasonable ground for supposing
that every movement, whether it be of the mass or among the constituent
particles, is attended by a change of electrical distribution; and if this
is true, it may easily be conceived that inasmuch as motion is the rule of
the universe, there must be a constant series of electrical changes. Now,
these changes do not all operate in one direction, nor are they all of
similar character, whence it is that not only are there earth currents of
feeble electro-motive force, but that this E.M.F. is constantly varying,
and that, furthermore, electricity of high E.M.F. is to be met with in
various parts of the atmosphere.
With earth currents we have here very little to do. The rotation of the
earth is in itself sufficient to generate small currents, and the fact
that they vary in strength at regular periods of the day and of the year
enforces the suggestion that the sun exerts considerable electrical
influence on the earth. Letting it be granted, however, that the earth is
variously charged, how comes it that the air is also charged, and with
electricity of greater tension than that of the earth itself? It was
pointed out by Sir W. Grove that if the extremities of a piece of platinum
wire be placed in a candle flame, one at the bottom and the other near the
top, an electric current will flow through the wire, indicating the
presence of electricity. If an electrified body be heated, the electricity
escapes more rapidly as the temperature rises. If a vessel of water be
electrified, and the water then converted into steam, the electric charge
will be rapidly dissipated. If a vessel containing water be electrified,
and the water allowed to escape drop by drop, electricity will escape with
each drop, and the vessel will soon be discharged.
We regard it as an established fact that the earth has always a greater or
less charge; whence it is safe to assume that in the process of
evaporation which is going on all over the surface of the globe, more
particularly in equatorial regions, every particle of water, as it rises
into the air, carries with it its portion, however minute that portion may
be, of the earth's electric charge. This small charge distributes itself
over the surface of the aqueous particle, and the vapor rises higher and
higher until it reaches that point above which the air is too rare to
support it. It then flows away laterally, and as it approaches colder
regions gets denser, sinking lower and nearer to the earth's surface. The
aqueous particles becoming reduced in size, the extent of their surfaces
is proportionately reduced. It follows that as the particles and their
surfaces are reduced, the charge is confined to a smaller surface, and
attains, therefore, a greater "surface density," or in simpler language, a
greater amount of electricity per unit of surface.
Electricity, as above set forth, is in what is known as the "static"
condition (to distinguish it from electricity which is being transferred
in the form of a current), when it has the property of "repelling itself"
to the utmost limits of any conductor upon which it may be confined. This
will account for the charge finding its way to the surface of the water
particles, and will furthermore account for the greater density of the
charge as the particle gets smaller and has the extent of its surface
rapidly diminished. It may be mentioned that the surface of a sphere
varies as the cube of its radius.
Returning to the discussion of the state of affairs existing when the
particles have reached their highest position in the atmosphere, we may
imagine that they set themselves off on journeys toward either the north
or the south pole. As they pass from the hotter to the colder regions, a
number of particles coalesce; these again combine with others on the road
until the vapor becomes visible as cloud. The increased density implies
increased weight, and the cloud particles, as they sail poleward, descend
toward the surface of the earth. Assuming that a spherical form is
maintained throughout, the condensation of a number of particles implies a
considerable reduction of surface. Thus, the contents of two spheres vary
as the cubes of their radii, or eight (the cube of 2) drops on combining
will form a drop twice the radius of one of the original drops. We may
safely conceive hundreds and thousands of such combinations to take place
until a cloud mass is formed, in which the constituent parts are more or
less in contact, and, therefore, behave electrically as a single conductor
of irregular surface, upon which is accumulated all the electricity that
was previously distributed over the surfaces of the millions of particles
that now compose it.
The tendency of an electric charge upon the surface of a conductor is to
take upon itself a position in which it may approach nearest to an equal
and opposite charge; or, if possible, to attain neutrality. If, then, a
cloud has a charge, and there is no other cloud above or near it, the
charge _induces_ on the adjacent earth surface electricity of the opposite
kind. Thus, assuming the cloud to be charged with positive electricity,
the subjacent earth will be in the negative state. The two
electricities[3] exert a strong tendency to combine or to produce
neutrality, whence there is a species of stress applied to the intervening
air. Possibly the cloud will be drawn bodily toward the earth more or less
rapidly, according as the charge is great or small. Or, on the other hand,
the cloud may roll on for leagues, carrying its influence with it, so that
the various portions of the earth underneath become successively charged
and discharged as the cloud progresses on its journey.
[Footnote 3: We may speak of two electricities or two electric states
without necessarily implying adherence either to the single or the double
"fluid" theory. Whether electricity be of two kinds or no, the fact
remains that there are two conditions, and all the features of this paper
may be explained with equal facility by the supporters of either
hypothesis.]
Should the cloud be near the earth, or should it be very highly charged,
the tension of the two electricities may be so great as to overcome the
resistance of the intervening air; and if this resistance should prove too
weak, what happens? How does the discharge show itself? It takes place in
the form of a lightning flash, and passing from the one surface to the
other--or, maybe, simultaneously from both--produces neutrality more or
less complete.
There has recently been a little discussion in these pages on the subject
of lightning, some having stated that they discerned the discharge to take
place upward--that is, from the earth toward the cloud. I will not venture
so far as to say whether or not the direction of the discharge is
discernible; possibly the flash may sometimes be long enough to enable one
to tell; but I have never so seen it, and have always looked upon the eye
as a deceitful member--very. "The lightning flash itself never lasts more
than 1/100000 of a second." It is, however, just as likely that a
discharge may travel upward as downward. What controls the discharge? Does
the quality of the charge?--that is to say, is the positive or the
negative more prone to break disruptively through the insulating medium?
Investigations with Geissler's and other tubes containing highly rarefied
gases have made it tolerably clear that there is a greater "tearing away"
influence at the negative than at the positive pole, and if two equal
balls, containing one a positive and the other a negative charge, be
equally heated, the negative is more readily dissipated than the positive.
But, so far as we at present know, this question enters into the
discussion scarcely, if at all. Our knowledge seems rather to point to the
substances upon which the charges are collected. The self-repellent nature
of electricity compels it to manifest itself at the more prominent parts
of the surface, the level being forsaken for the point. The tension of the
charge, or its tendency to fly off, is proportionately increased. And if
at a given moment the tension attains a certain intensity, the discharge
follows, emanating from the surface which offers the greatest facilities
for escape. The earth is generally flatter than the cloud, whence, in all
probability, the discharge more frequently originates with the cloud.
Should a lightning flash strike the earth and produce direct neutrality,
it is possible that no damage will result, although this again is not
always certain, because when the cloud charge acts inductively on the
earth it produces the opposite (say negative) charge on the nearer parts,
the similar (or positive) state is also produced at some place more or
less distant. Sometimes this "freed" positive (which, by the way,
accumulates gradually and physiologically imperceptibly) is collected at
some portion of the earth's surface. When the negative is neutralized by
the discharge, the freed positive is no longer confined to a particular
region, but tends to dissipate itself, and a shock may be felt more or
less severely by any person within the region. Or, again, a similar shock
may be experienced by a person standing within the negative zone on the
neutralization of the charge.
I may take the opportunity here to mention a highly interesting and
instructive incident observed on local telegraph circuits during a
thunderstorm. The storm may be taking place at some distance from the
point of observation. The electrified cloud induces the opposite charge
beneath it, the similar charge being repelled. It is noticeable that the
needle of a galvanometer, starting from the middle position, goes
gradually over to one side, eventually indicating a considerable
deflection. Suddenly, owing apparently to a lightning discharge some
distance away, the force which caused the deflection is withdrawn, and the
needle rebounds with great violence to the opposite side. In a short time,
the cloud becoming again charged on its under surface, and recommencing
its inductive effect upon the adjacent earth, the needle starts again, and
goes through the same series of movements, a violent counterthrow
following every flash of lightning.
If we can so far control our imagination, we may conceive the earth to be
one large insulated conductor, susceptible to every influence around it.
If then the earth, as a mass of matter, behaves as above indicated, there
is no plausible reason for declining to regard any other large conducting
mass in a similar light, and as a body capable of being subjected more or
less completely to the various impulses affecting the earth. In other
words, a large mass of conducting material, partially or perfectly
insulated, is, during a thunderstorm, in considerable danger. With this
portion of the subject I shall, however, deal more fully when discussing
the merits of lightning protectors.
Lightning discharges do not take place between cloud and earth only, but
also, and perhaps more frequently, between too oppositely charged clouds.
We then get atmospheric lightning, the flash often extending for miles.
This form of lightning is harmless, and in all probability what we see is
only a reflection of the discharge. The oft-told tale of the lightning
flying in at the window, across the room, and out of the door, or up the
chimney, is all moonshine, and before dealing with lightning protectors I
intend to expose some of the fallacies concerning lightning. Were the
discharge to pass through a house, it would infallibly leave more decided
traces and do more damage than simply scaring a superstitious old lady now
and again. Many people are often and unnecessarily frightened during a
thunderstorm, but it may be safely predicted that a person under a roof is
infinitely safer than one who is standing alone on level ground, and
making himself a prominence inviting a discharge. Rain almost invariably
accompanies the discharge, and the roof and sides of the house being wet,
they form a more or less perfect channel of escape should a flash strike
the building.--_Knowledge._
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