ATMOSPHERIC ELECTRICITY
Every schoolboy has read how Benjamin Franklin, by means of his famous
kite experiment, demonstrated the electrical nature of lightning, and
how the same versatile genius invented the lightning rod. It is not
proposed to repeat familiar history here. Neither shall we discuss
the dubious statements frequently put forth that lightning rods were
known before Franklin’s time, nor consider how much credit is due
the many philosophers who, at earlier periods, suspected lightning
to be a manifestation of electricity. The facts and ideas concerning
atmospheric electricity that we have to present in this chapter were,
for the most part, quite unknown to Franklin and to many generations of
_savants_ after him, and some of them are just now finding their way
into the textbooks.
Science still recognizes the existence of two kinds of
electricity--positive and negative--which, by combining, neutralize
each other’s effects. According to current ideas, however, the more
active agent in electrical phenomena is negative electricity, which
is believed to consist of (or to provide electrical charges for)
exceedingly minute particles called _electrons_.
Only a few years ago the smallest thing that science had to deal
with was the atom, and the lightest of atoms is that of hydrogen.
The discovery of electrons marks a new step toward the infinitely
little. The mass of the electron--or, in more popular and less exact
terms, its weight--is about 1/1800 that of a hydrogen atom. As to its
size: Imagine a billiard ball magnified to the size of the earth. Its
constituent atoms would be the actual size of billiard balls, but the
electrons of which each atom is composed would still be too small to be
seen with the naked eye. Now imagine each of these billiard-ball atoms
further magnified to the size of a large church. The electrons would
then be about as big as one of the periods on this page.
When we say that a body has an electrical charge we mean that it has
an _excess_ of positive or negative electricity. An ordinary molecule
of an atmospheric gas contains (or perhaps actually consists of)
equal amounts of the two kinds of electricity, and is therefore not
charged. There are, however, various ways in which an electron may
be detached from such a molecule, leaving it positively charged; and
again it may receive an extra electron, and thus acquire a negative
charge. Under the former circumstances it becomes a _positive ion_,
and under the latter a _negative ion_. Ions play a very important role
as carriers of electricity, because they are impelled to move toward
oppositely charged bodies or particles and combine with them. A gas
containing ions is said to be _ionized_; and it is the ionization of
the atmosphere that makes it a conductor of electricity.
The number of ions in a given volume of air has been the subject
of a great many measurements, both at observatories on land and in
the course of scientific expeditions at sea. There are ingenious
instruments called “ion counters,” in which air is drawn at a measured
rate through the apparatus and its electrical effects are noted.
The number of positive ions found in a cubic centimeter of the lower
atmosphere varies from a few hundred to a thousand or more, while the
number of negative ions in the same space is generally one or two
hundred smaller. The ionization is about the same over the ocean as
over the land.
There are several ways in which the air may become ionized. The
different rays given off by radioactive substances (Alpha, Beta, and
Gamma rays) all have the power of driving off electrons from the
molecules of gases; i. e., ionizing them. Air is undoubtedly ionized by
radioactive matters in the soil (radium and thorium) and especially by
the gaseous “emanations” of these substances in the atmosphere, which
are also radioactive. It has, however, been a problem to account for
ionization over the ocean; because the amount of radioactive matter
in sea water is immeasurably small, while the amount of radioactive
emanation in sea air is, according to the latest observations, only
about 2.5 per cent of that occurring over the land.
The clue to this mystery seems to be found in a special kind of Gamma
rays coming from some region far above the surface of the earth. These
rays are called the “penetrating radiation,” because they not only are
able, like the Gamma rays due to radioactive substances on earth, to
pass through the walls of hermetically sealed metal vessels and ionize
the air inside, but they also have the power of passing through a great
extent of atmosphere without being absorbed. They are estimated to be
about ten times as “penetrating” as the Gamma rays coming from known
terrestrial substances. The best proof that a radiation of this sort
comes from above is that when closed metal vessels are carried up in
balloons, there is, above an altitude of about half a mile, a rapid
increase in the rate at which ions are produced within them. As to the
source of this radiation, one suggestion has been that it comes from
strongly radioactive cosmic dust in the upper atmosphere. A hypothesis
that seems more plausible at present attributes it to the bombardment
of the atmosphere by electrons shot off from the sun.
The knowledge of ions in the atmosphere is one of the recent
acquisitions of science. On the other hand, it has been known for
some generations that the earth has normally a negative charge as
compared with the air, or the air a positive charge as compared with
the earth. Thus between the earth and any point in the air (except,
as we now know, at great altitudes) there is a difference of what is
called “potential,” of such a character that negative electricity will
follow any conductor provided for it away from the earth. Variations
of potential with altitude have long been measured by means of
instruments called “collectors,” which gather, so to speak, a sample
of the electrical charge of the air at any point and enable it to
be compared with that of the earth. The difference of potential is
measured in volts per meter of vertical distance. Thus we get the
“potential gradient,” which averages about 150 volts per meter in
the lowest part of the atmosphere. It is subject to great irregular
variations--especially during thunderstorms--and also to somewhat
regular rises and falls during each day, and to an annual fluctuation,
being much greater in winter than in summer.
It has also been known for a good many years that the air is a
conductor of electricity--though a poor one--and, therefore, does not
insulate the earth. Dr. W. F. G. Swann has expressed the extremely
small conductive capacity of the air for electricity in the statement
that a column of it one inch long offers as much resistance to the
passage of an electrical current as a copper cable, of the same cross
section, thirty thousand million million miles long!
A fact more recently learned, from observations in balloons, is that
the potential gradient falls off rapidly at high levels, and becomes
practically zero at an altitude of about six miles. From this fact
it is concluded that the lower six miles of the atmosphere contains
a charge of positive electricity just equal to the negative charge
at the earth’s surface. In other words, the lower atmosphere is not
only positive with respect to the earth, but in an absolute sense it
contains an excess of positive electricity.
Thus we have a negatively charged earth surrounded by a layer of
positively charged air. Since air is a conductor, it is not easy to see
why the opposite charges of the earth and the atmosphere do not combine
and neutralize each other. An interchange is, in fact, always going on
between them; negative ions flow upward from the earth and positive
ions flow in the opposite direction. This “earth-air current” is,
however, exceedingly small. Moreover, the opposite charges of the earth
and air remain from year to year in spite of it.
How does the earth retain its negative charge and the air its positive
charge? No other question relating to atmospheric electricity has, in
recent years, been so much debated as this. Discussion centers, as a
rule, upon the negative charge of the earth; for there are certain
reasons for assuming that, when once this is explained, the positive
charge of the atmosphere will require no special explanation.
One hypothesis is that the earth is bombarded by positive and negative
corpuscles from the sun, and that the negative corpuscles have such
penetrating power that they are able to reach the earth, while the
positive corpuscles are caught by the atmosphere. Another hypothesis
(Swann’s) is that the same “penetrating radiation” that, as we have
seen, helps to ionize the lower atmosphere has the effect of driving
downward a stream of electrons detached from the air molecules, thus
maintaining a constant supply of negative electricity to the earth. The
question is not yet settled.
It is now time to turn from these somewhat abstruse matters to a
subject of universal interest; viz., lightning. As recently as a few
decades ago, though there was already a copious literature on the
subject of lightning, very little was really known about it. Even its
superficial features were strikingly misunderstood until the advent of
photographic methods of investigation. Thus until the middle of the
nineteenth century sharply zigzag lightning flashes were represented
in scientific books as they still are in conventional art. That
so-called zigzag lightning is really sinuous was first asserted by
James Nasmyth, in 1856, and his contention was soon afterward confirmed
by photography. The camera has revealed a large number of other
interesting things about lightning.
Everybody has noticed an appearance of flickering in lightning flashes
that are of sensible duration. Several early investigators, such as
Arago, Dove, and O. N. Rood, had reached the conclusion that such
flashes are multiple, consisting of several successive discharges
along an identical path. Rough measurements of the intervals of time
between these discharges were made with various forms of rotating disk.
Far more accurate information is now obtained on this subject by the
use of a camera mounted on a vertical axis and swung in a wide arc, at
a fixed rate, by clockwork. The perfection of this device is due, in
part, to A. Larsen, in America, but especially to Dr. B. Walter, of
Hamburg, whose achievements in the photography of lightning far surpass
those of any other investigator.
It is obvious that if a discharge of lightning is not instantaneous,
but has a sensible duration, the rotary movement of the camera,
arranged as just mentioned, will spread out the image of the flash,
on the photographic plate, into a more or less broad band or ribbon.
Most photographs of ribbonlike streaks of lightning made with ordinary
cameras are, in fact, due to accidental movements of the apparatus
during exposure--such as an involuntary start of the operator, in case
the camera is held in the hands--though a certain amount of spreading
of the image is sometimes caused by what photographers call “halation.”
Pictures taken with the revolving camera show that some flashes are
practically instantaneous while others may last as long as half a
second or more. Those of the latter class nearly always show several
parallel streams of light, more or less distinctly separated by darker
spaces. Each of these bright streams represents a separate discharge
along the common path. As the speed with which the camera turns is
known, it is possible to determine the intervals of time between the
discharges of a multiple flash. These intervals may vary from a few
thousandths to one or two-tenths of a second, while the duration of
each of the consecutive discharges is probably not more than two or
three hundred-thousandths of a second in most cases. Sometimes the path
of the lightning flash is shifted by the wind while the picture is
being taken. In one case this shift was estimated at 36 feet.
Photography is also applied to determining the distance of a lightning
flash and hence the dimensions of any of its features. For this purpose
a stereoscopic method is used, two cameras being mounted side by side
and exposed at the same time. Sometimes one of the cameras is made to
revolve, while the other remains stationary. The stationary camera
will then show the relative positions of the flashes occurring during
exposure, while the moving camera will indicate the times at which they
occurred.
Streaks of “black lightning” and black borders of the white flashes,
both often seen in photographs, are a trick of the camera and are due
to what is called the “Clayden effect.” Some kinds of plates are much
more susceptible to this effect than others. When a flash of lightning
has registered its impression on such a plate, and, before the shutter
is closed, another flash occurs, the general illumination of the field
by light reflected from clouds, etc., often “reverses” the original
image, and consequently it prints black.
“Sheet lightning” presents the appearance of a diffuse glow over the
sky. When lightning of this character is seen playing about the horizon
on summer evenings, in the absence of an audible thunderstorm, it is
often called “heat lightning.” Most sheet lightning is probably a mere
reflection of ordinary streak lightning below the horizon or hidden
by clouds. Some authorities believe, however, that diffuse, silent
discharges actually occur in the clouds. Balloonists claim to have
encountered such discharges near at hand. An analogous phenomenon is
the glowing of so-called “incandescent” or self-luminous clouds, to
which several observers have called attention. A remarkable phenomenon
of somewhat similar character has been reported by Dr. Knoche, late
director of the Chilean meteorological service, who states that it
occurs on a vast scale along the crest of the Andes during the warm
season. The mountains seem to act as gigantic lightning rods, giving
rise to more or less continuous diffuse discharges between themselves
and the clouds, with occasional outbursts simulating the beams of
a great searchlight. These displays are visible hundreds of miles
out at sea. Something akin to this so-called “Andes lightning” has
occasionally been reported from other mountainous regions, including
the mountains of Virginia and North Carolina.
“Beaded” lightning and “rocket” lightning are as rare as they are
interesting. The former resembles a string of glowing beads, while the
latter is a form of streak lightning that shoots up into the air at
about the apparent speed of a skyrocket.
“Ball lightning” takes the form of a fiery mass (not always globular),
which generally moves quite deliberately through the air or along the
ground, and in many cases disappears with a violent detonation. It
occurs inside of buildings, as well as out of doors.
In order that a discharge of electricity may break through the
resistance of the air along paths as long as those commonly observed,
enormous differences of potential must exist in the atmosphere during
thunderstorms. How such conditions arise has been the subject of
an immense amount of speculation. The explanation now generally
accepted was proposed in the year 1909 by the English physicist and
meteorologist, Dr. George Simpson. This hypothesis is based upon the
fact, well attested by laboratory experiments, that the breaking up
of drops of water involves a separation of positive from negative
electricity; in other words, the production of both positive and
negative ions. In this process the drops become positively charged;
i. e., they retain a greater number of positive than of negative
ions, the latter being set free in the air. About three times as many
negative as positive ions are thus released.
Now a thunderstorm is accompanied by strong upward movements of the
air; so strong that small drops cannot fall to the ground, while large
drops, which would be heavy enough to fall through such rising currents
if they could retain their integrity, are broken up by the blast of air
and carried aloft, where they tend to accumulate, recombine, and fall
again. This process may be repeated many times, so that the positive
charge of the drops is continually increasing, and at the same time
negative ions are being set free and carried by the ascending air to
the upper part of the clouds. Here they unite with the cloud particles
and give them a strong negative charge. Thus eventually there is formed
a heavily charged positive layer of cloud between a heavily charged
negative layer above and the negatively charged earth beneath. When
the differences of potential thus brought about become great enough,
disruptive discharges of electricity will occur, and these may be
either between the upper and lower layers of cloud or between the
clouds and the earth, or, sometimes, between two different clouds.
Probably much the most frequent lightning flashes are those that occur
within a single thundercloud and do not reach the earth. However, it
will often happen that the negatively charged upper layer of cloud is
either carried very high or drifted away by the wind, and then the
discharges that occur will be chiefly between cloud and earth. Such
conditions are likely to prevail in the case of cyclonic thunderstorms,
in which there is often great difference in the direction and force of
the winds at different levels. On the other hand, heat thunderstorms
usually occur when the general winds are light at all levels, and it
is probable that such storms are relatively free from cloud-to-earth
discharges. We seem to have here an explanation of the paradox that
tropical thunderstorms, which are nearly always of the noncyclonic
type, though notoriously violent, are generally harmless.
It must not be inferred from what has been said above that the mystery
of the lightning flash is now fully resolved. This is far from being
the case. It is not at all clear how an electrical discharge can break
down the resistance of the air along a path a mile or more in length,
as commonly happens in the thunderstorm. It was formerly stated, on
good authority, that the difference of potential required to produce
such a flash would amount to upward of 5,000,000,000 volts. Certain
facts have lately been adduced to show that such great differences
of potential need not be assumed. Moving-camera photographs of the
sparks produced by electric machines show that such sparks begin with
small brush discharges which gradually ionize the air and thus build
up a conductive path for the complete discharge. Something of this
kind may occur in the atmosphere. Streaks of air already strongly
ionized and more or less continuous sheets of rain would also help to
provide conductors for a discharge. If lightning does build up its
path somewhat gradually, the process might, in certain cases, be so
slow as to account for the deliberate movement of rocket lightning,
and also, perhaps, furnish a clue to the hitherto unsolved mystery of
ball lightning. Humphreys has tentatively suggested that all genuine
cases of ball lightning are “stalled thunderbolts”; i. e., lightning
discharges that have come to a halt, or nearly so, in their progress
through the air.
As to the visibility of lightning Humphreys says, in his “Physics of
the Air”:
“Just how a lightning discharge renders the atmosphere through which it
passes luminous is not definitely known. It must and does make the air
path very hot, but no one has yet succeeded, by any amount of ordinary
heating, in rendering either oxygen or nitrogen luminous. Hence it
seems well-nigh certain that the light of lightning flashes owes its
origin to something other than high temperature, probably to internal
atomic disturbances induced by the swiftly moving electrons of the
discharge, and to ionic recombination.”
A few attempts have been made to measure the strength of current in a
lightning discharge. Many substances become magnetized when an electric
discharge occurs in their vicinity, and it has been pointed out by F.
Pockels that when basalt rock is magnetized in this way the amount of
magnetism is an indication of the greatest strength of current to which
it has been exposed. Pockels examined specimens of basalt from the top
of Mount Cimone, in the Apennines, where lightning strokes are common,
and found many of them more or less magnetized. He also exposed blocks
of basalt close to a branch of a lightning rod in the same region.
He thus obtained values for the strength of current in lightning
discharges ranging from 11,000 to 20,000 amperes. Humphreys, from the
crushing effect of a lightning stroke upon a hollow lightning rod,
has computed the strength of current in the case examined to be about
90,000 amperes.
The effects of lightning are so various that it would take a book to
describe them all. Its audible effects are discussed in our chapter
on atmospheric acoustics. Its chemical effects consist chiefly in the
production of oxides of nitrogen, ozone, and probably ammonia from
the constituents of the atmosphere along the path of the discharge,
and these substances, either directly or after further combinations
in the atmosphere, contribute to the fertility of the soil. Lightning
sometimes bores holes several feet deep in sandy ground and fuses
the material along its path, forming the glassy tubes known as
_fulgurites_. Similar holes are bored in solid rock.
The destructive effects of lightning are due chiefly to the heat
generated by the passage of an electric current through a poor
conductor. When moisture is present in the object struck, its sudden
conversion into steam produces the explosive effects seen in the
shattering of trees, the ripping of clothes from the human body, etc.
There is almost no end to the curious pranks played by lightning--some
disastrous, some comical, and some benevolent, as when persons crippled
with rheumatism, after having been knocked down and temporarily stunned
by a stroke of lightning, have found themselves completely cured of
their malady! A well-known book by Camille Flammarion, translated
into English under the title “Thunder and Lightning,” is almost wholly
devoted to these eccentricities of the lightning stroke.
[Illustration: A LIGHTNING PRINT ON THE ARM OF A BOY STRUCK BY
LIGHTNING NEAR DUNS, SCOTLAND, IN 1883
_Drawn from a photograph taken a few hours after the accident__. From
the Lancet._]
There is a very common belief that lightning sometimes impresses a
photographic image of trees or other objects of the landscape upon the
human body. The ramifying pink marks, known as “lightning prints,”
often found on the skin of persons who have been struck by lightning,
are, however, in no sense photographic, but are merely the lesions due
to the passage through the tissues of a branching electric discharge.
A few practical suggestions in regard to danger from lightning are
offered by Humphreys, as follows:
“Generally it is safer to be indoors than out during a thunderstorm,
and greatly so if the house has a well-grounded metallic roof or
properly installed system of lightning rods. If outdoors it is far
better to be in a valley than on the ridge of a hill, and it is always
dangerous to take shelter under an isolated tree--the taller the tree,
other things being equal, the greater the danger. An exceptionally tall
tree is dangerous even in a forest. Some varieties of trees appear to
be more frequently struck, in proportion to their numbers and exposure,
than others, but no tree is immune. In general, however, the trees
most likely to be struck are those that have either an extensive root
system, like the locust, or deep tap roots, like the pine, for the very
obvious reason that they are the best grounded and therefore offer, on
the whole, the least electrical resistance.
“If one has to be outdoors and exposed to a violent thunderstorm, it
is advisable, so far as danger from lightning is concerned, to get
soaking wet, because wet clothes are much better conductors, and dry
ones poorer, than the human body. In extreme cases it might even be
advisable to lie flat on the wet ground. In case of severe shock,
resuscitation should be attempted through persistent artificial
respiration and prevention from chill.
“The contour of the land is an important factor in determining the
relative danger from lightning because the chance of a discharge
between cloud and earth, the only kind that is dangerous, varies
somewhat inversely as the distance between them. Hence thunderstorms
are more dangerous in mountainous regions, at least in the higher
portions, than over a level country. Clearly, too, for any given region
the lower the cloud the greater the danger. Hence a high degree of
humidity is favorable to a dangerous storm, partly because the clouds
will form at a lower level and partly because the precipitation, and
probably therefore the electricity generated, will be abundant. Hence,
too, a winter thunderstorm, because of its generally lower clouds, is
likely to be more dangerous than an equally heavy summer one.”
It is estimated that the total property loss due to lightning in the
United States is about $8,000,000 a year, and the number of persons
struck about 1,500, of whom one-third are killed. Nine-tenths of these
accidents occur in rural localities.
Lightning rods neither prevent lightning stroke nor do they, as is
sometimes alleged, attract lightning to buildings. They merely provide
good conductors along which a stroke of lightning may reach the earth
without doing damage, and, within very moderate limits, determine the
path of discharge. While there are many unsettled points regarding the
theory of lightning rods and details of construction, their general
utility is strikingly indicated by statistics showing the comparative
amount of damage done by lightning to rodded and unrodded buildings.
According to the United States Bureau of Standards, information
gathered in this country shows that “taking rods as they come in
the general run of installations, they reduce the fire hazard from
lightning by 80 to 90 per cent in the case of houses, and by as much
as 99 per cent in the case of barns.” The same bureau, in its valuable
publication, “Protection of Life and Property Against Lightning”
(Washington, 1915), supplies the answers to a multitude of questions
that are constantly asked about lightning rods.
Buildings with metal roofs and frames connected with the ground are
generally well protected from lightning (except as to nonmetallic
chimneys) without rods.
During actual thunderstorms, and also at other times when there
are high potential gradients in the atmosphere, luminous electric
discharges of a more or less continuous character are sometimes
observed to occur in the shape of small jets and flames, chiefly from
pointed objects, including lightning rods, the masts and spars of
vessels, the angles of roofs, etc. These are identical in character
with the “brush” discharges, or incomplete sparks, produced by electric
machines. They are accompanied by a hissing or crackling sound. Their
luminosity is comparatively feeble, and for this reason they are much
more often observed by night than by day. They are especially common
during snowstorms.
This phenomenon is known as _St. Elmo’s fire_ or _corposants_ (not to
mention a score of other names, ancient and modern). As seen at sea,
corposants sometimes take the form of one or two starlike objects at
the trucks of the masts or the ends of yard arms, but occasionally the
spars, rigging and other parts of the ship are lighted up with a great
number of stationary or moving flames, producing a weird spectacle. The
finest examples of corposants are, however, observed on high mountains,
and the phenomenon has been carefully studied at certain mountain
observatories, such as those on Ben Nevis and the Sonnblick.
Of its occurrence on Ben Nevis, Angus Rankin writes: “The most frequent
manner in which it makes its appearance is as caps of light on the tips
of the lightning rod, but occasionally it appears as jets of flame
projecting from all objects on the top of the tower and from the cowl
of the kitchen chimney, which rises from the roof at some distance
from the tower. These jets are at times from 4 to 6 inches in length,
and make a peculiar hissing sound. During a very brilliant display,
the observer’s hair, hat, pencil, etc., are aglow with the ‘fire,’
but, except for a slight tingling sensation in the head and hands, he
suffers no inconvenience from it. On such occasions, if a stick be
raised above the head, jets of electric light will be seen at its upper
end. The only drawback to observing it with advantage is the unpleasant
character of the weather in which it appears, namely blinding showers
of snow and hail, and squally winds, causing a good deal of snowdrift.”
Rankin records that it was sometimes _heard_ in the daytime, when its
light was invisible. On the Sonnblick a display of St. Elmo’s fire has
been observed to last as long as eight hours.
No luminous electrical phenomenon is more beautiful or, at first sight,
more mysterious than the _aurora_, popularly known, in the northern
hemisphere, as the “northern lights.” This phenomenon is due to the
passage of electrical discharges through the rarefied gases of the
upper atmosphere, and it now appears to be settled beyond controversy
that the discharges are caused by corpuscles or radiations of some kind
emitted from the sun.
Most accounts of the aurora describe the typical appearances that it
assumes as seen from a single place on the surface of the earth, but
say little, if anything, about the form of the phenomenon as a whole
or about its position in space. We shall follow a different plan here,
and ask the reader, first of all, to imagine himself viewing the aurora
from a point some thousands of miles away from our planet.
The solar emission above mentioned, when sufficiently intense, produces
in the upper atmosphere a glow like that seen in a vacuum tube when
an electrical discharge passes through it. From our vantage point in
outer space we shall notice that this glow is not spread over the
whole globe, but forms two rings, which encircle the polar regions of
both hemispheres, though neither the geographic nor the magnetic poles
lie at their centers. The rings do not extend down into the lower
atmosphere, but hang about 60 miles above the earth’s surface.
The reason for this segregation of the aurora in high latitudes is
that the earth is a great magnet, and magnets have the power of
deflecting an electrical discharge in their vicinity. An appearance
much resembling the two auroral zones of the earth was produced, on
a small scale, by the late Professor Kr. Birkeland of Christiania,
who magnetized a metal globe and allowed an electrical discharge to
play upon it in a vacuum. The surface of the globe was coated with
a phosphorescent substance, which glowed under the discharge in two
rings, corresponding roughly to those of the aurora. In both cases the
discharge follows what are called the magnetic “lines of force.” Our
earth, like other magnets, is enveloped and penetrated by such lines.
At any point on the earth the direction of the neighboring lines of
force is shown by the dipping needle, which assumes a position parallel
to them. At a magnetic pole the needle points straight up and down,
and everywhere in high latitudes it has a position not very much
inclined to the vertical, while in low latitudes it is more or less
horizontal.
If, now, for the sake of simplicity we confine our attention to the
northern hemisphere, and imagine ourselves maintaining our watch for
months and years together, we shall discover that much of the time
there is no ring to be seen; at other times there may be a small or
partial ring; and occasionally there is a very broad, conspicuous ring,
spreading so far south that it overlies the northern part of the United
States and most of Europe. Evidently the emission from the sun that
causes the auroral discharge varies greatly in strength, and this is in
accordance with what we know about solar activities in general.
Next let us take a closer look at the ring, whether from outer space or
from the earth’s surface. We shall find that it is made up, at least
in part, of a multitude of luminous beams directed out into space and
undergoing rapid changes in position and form. These beams, which
really mark out the streams of the discharge in the upper air, follow
the lines of force. In high latitudes they are nearly vertical with
respect to the underlying surface of the earth. Even in the United
States (when the aurora extends so far south) they are much more nearly
vertical than horizontal. A dipping needle will show, at any place,
just how they should stand.
[Illustration: PHOTOGRAPHIC ANALYSIS OF LIGHTNING. This double
photograph was made by Dr. B. Walter of Hamburg, the leading expert in
the field of lightning photography. The picture at the left was taken
with a stationary camera. The photograph at the right, taken at the
same time with a revolving camera, shows that one of the main flashes
(the one to the right) was a single discharge, and the other a multiple
discharge.]
From any distant point on the earth’s surface either north or south of
it, the visible portion of the auroral ring presents the appearance
of an arch across the horizon. Arctic explorers, far within the
Arctic Circle, see this arch to the south of them. In our latitudes
it spans the northern horizon. Separate beams or streamers may be
distinguishable or not, according to the brightness of the discharge
or its distance from the point of observation. Combinations of beams
constitute so-called “draperies.”
[Illustration: PHOTOGRAPH OF LIGHTNING, SHOWING “BLACK” FLASHES.
(_graph by F. Ellerman._)]
[Illustration: ATMOSPHERIC ELECTRICITY INSTRUMENTS ON BOARD
THE “CARNEGIE.” Left: Penetrating radiation apparatus. Right:
active-content apparatus. Below: Arrangement for supplying potentials
to electroscopes and ionization chambers, (_Carnegie Institution._)]
Occasionally, at times of great solar activity, part of the ring
actually overlies our Northern States, and the aurora then becomes a
magnificent spectacle in this part of the world. The whole sky may be
filled with the shifting streamers, along which travel rapid pulsations
of light, so that the phenomenon then suggests strongly what it really
is--a vast electrical discharge passing down through the atmosphere
from outer space. When the observer is thus surrounded by the beams,
they seem, on account of perspective, to converge toward a point south
of the zenith, where they form a beautiful _corona_ or crown. The
position of this crown depends upon the slant of the beams, which, as
already explained, follow the lines of force.
A brilliant aurora is always accompanied by disturbances of the
magnetic needle, which moves about erratically, so that compasses can
no longer be depended upon. At the same time there are strong “earth
currents,” which interfere with the operation of telegraph lines.
Observations with the spectroscope seem to show that the light of the
aurora is chiefly due to glowing nitrogen, though the most prominent
line in the auroral spectrum has sometimes been referred to an
unknown atmospheric gas. The various colors seen in bright auroras,
including reds, greens, and yellows, are believed by some authorities
to depend upon the varying speed of the electrical discharge.
Experiments with vacuum tubes show that nitrogen, especially, gives
great changes of color with changes in the velocity of the discharge.
Another interesting revelation of the spectroscope is that there is
apparently, a faint auroral illumination in the sky at all times and in
all parts of the world, the so-called “permanent aurora.”
Photography has been used with great success in studying the aurora,
especially by the Norwegian physicists Störmer, Vegard, and Krogness.
Simultaneous photographs of a single detail are taken from two points
several miles apart against a background of stars. The apparent
position of the auroral detail among the stars will differ in the two
pictures, and a comparison of them makes it possible to determine the
actual position of the aurora in space. A slow-moving cinematograph has
also been used to obtain series of pictures. The measurements of these
observers show that the base of the aurora is, generally between 60 and
70 miles above the earth with a strong tendency to assume a definite
location at an altitude of about 61 or 67 miles. Its upper limits are
not well defined, but it has been photographed up to an altitude of
more than 300 miles. Earlier observers reported seeing the aurora at
altitudes of only a few miles, and even down to the earth’s surface,
but recent authorities are inclined to discredit these observations.
One more phenomenon of atmospheric electricity requires brief mention,
viz., the electric waves that produce the erratic disturbances known to
wireless telegraph operators as “strays” or “static.” As heard in the
receiver of a wireless outfit the noise of strays has been described as
“like hailstones beating against a sheet of tin,” or “short hisses from
a steam pipe,” or “periodic discharges of coal down a chute.” Another
characteristic sound is a sharp “click.” The study of strays has been
carried out on a world-wide scale by a committee of the British
Association for the Advancement of Science, but their nature is not yet
fully understood. Some strays are undoubtedly due to near or distant
discharges of lightning, and special forms of wireless apparatus, known
as “thunderstorm recorders” or “ceraunographs,” have been used to give
notice of the approach of thunderstorms. On the other hand, strays
seem frequently to have no connection with thunderstorms, and their
principal origin is now sought in electrical disturbances in the upper
atmosphere, perhaps similar to those which cause the aurora, and, as in
the case of the aurora, having their ultimate source in the sun.