ELECTRICITY IN THE ATMOSPHERE
If the Leyden jars of a Wimshurst machine are connected up and the
discharging balls placed at a suitable distance apart, the electricity
produced by rotating the plates is discharged in the form of a
brilliant zigzag spark between the balls, accompanied by a sharp crack.
The resemblance between this spark and forked lightning is at once
evident, and in fact it is lightning in miniature. The discharging
balls are charged, as we have seen, with opposite kinds of electricity,
and these charges are constantly trying to reach one another across
the intervening air, which, being an insulator, vigorously opposes
their passage. There is thus a kind of struggle going on between the
air and the two charges of electricity, and this keeps the air in
a state of constant strain. But the resisting power of the air is
limited, and when the charges reach a certain strength the electricity
violently forces its way across, literally rupturing or splitting the
air. The particles of air along the path of the discharge are rendered
incandescent by the heat produced by the passage of the electricity,
and so the brilliant flash is produced. Just as a river winds about
seeking the easiest course, so the electricity takes the path of least
resistance, which probably is determined by the particles of dust in
the air, and also by the density of the air, which becomes compressed
in front, leaving less dense air and therefore an easier path on each
side.
The connexion between lightning and the sparks from electrified bodies
and electrical machines was suspected by many early observers, but it
remained for Benjamin Franklin to prove that lightning was simply a
tremendous electric discharge, by actually obtaining electricity from a
thunder-cloud. Franklin was an American, born at Boston in 1706. He was
a remarkable man in every way, and quite apart from his investigations
in electricity, will always be remembered for the great public
services he rendered to his country in general and to Philadelphia
in particular. He founded the Philadelphia Library, the American
Philosophical Society, and the University of Pennsylvania.
Franklin noticed many similarities between electricity and lightning.
For instance, both produced zigzag sparks, both were conducted by
metals, both set fire to inflammable materials, and both were capable
of killing animals. These resemblances appeared to him so striking that
he was convinced that the two were the same, and he resolved to put the
matter to the test. For this purpose he hit upon the idea of using a
kite, to the top of which was fixed a pointed wire. At the lower end of
the flying string was tied a key, insulated by a piece of silk ribbon.
In June 1752, Franklin flew his kite, and after waiting a while he
was rewarded by finding that when he brought his knuckle near to the
key a little spark made its appearance. This spark was exactly like
the sparks obtained from electrified bodies, but to make things quite
certain a Leyden jar was charged from the key. Various experiments were
then performed with the jar, and it was proved beyond all doubt that
lightning and electricity were one and the same.
Lightning is then an enormous electric spark between a cloud and the
Earth, or between two clouds, produced when opposite charges become so
strong that they are able to break down the intervening non-conducting
layer of air. The surface of the Earth is negatively electrified,
the electrification varying at different times and places; while the
electricity of the air is usually positive, but frequently changes to
negative in rainy weather and on other occasions. As the clouds float
about they collect the electricity from the air, and thus they may be
either positively or negatively electrified, so that a discharge may
take place between one cloud and another, as well as between a cloud
and the Earth.
Lightning flashes take different forms, the commonest being forked or
zigzag lightning, and sheet lightning. The zigzag form is due to the
discharge taking the easiest path, as in the case of the spark from
a Wimshurst machine. Sheet lightning is probably the reflection of a
flash taking place at a distance. It may be unaccompanied by thunder,
as in the so-called “summer lightning,” seen on the horizon at night,
which is the reflection of a storm too far off for the thunder to be
heard. A much rarer form is globular or ball lightning, in which the
discharge takes the shape of a ball of light, which moves slowly along
and finally disappears with a sudden explosion. The cause of this form
of lightning is not yet understood, but it is possible that the ball of
light consists of intensely heated and extremely minute fragments of
ordinary matter, torn off by the violence of the lightning discharge.
Another uncommon form is multiple lightning, which consists of a number
of separate parallel discharges having the appearance of a ribbon.
A lightning flash probably lasts from about 1/100,000 to 1/1,000,000 of a
second, and in the majority of cases the discharge is oscillatory; that
is to say, it passes several times backwards and forwards between two
clouds or between a cloud and the Earth. At times it appears as though
we could see the lightning start downwards from the cloud or upwards
from the Earth, but this is an optical illusion, and it is really
quite impossible to tell at which end the flash starts.
Death by lightning is instantaneous, and therefore quite painless.
We are apt to think that pain is felt at the moment when a wound is
inflicted. This is not the case however, for no pain is felt until the
impression reaches the brain by way of the nerves, and this takes an
appreciable time. The nerves transmit sensations at a speed of only
about one hundred feet per second, so that in the case of a man killed
by a bullet through the brain, no pain would be felt, because the brain
would be deprived of sensibility before the sensation could reach it.
Lightning is infinitely swifter than any bullet, so life would be
destroyed by it before any pain could be felt.
On one occasion Professor Tyndall, the famous physicist, received
accidentally a very severe shock from a large battery of Leyden jars
while giving a public lecture. His account of his sensations is very
interesting. “Life was absolutely blotted out for a very sensible
interval, without a trace of pain. In a second or so consciousness
returned; I saw myself in the presence of the audience and apparatus,
and, by the help of these external appearances, immediately concluded
that I had received the battery discharge. The intellectual
consciousness of my position was restored with exceeding rapidity,
but not so the optical consciousness. To prevent the audience from
being alarmed, I observed that it had often been my desire to receive
accidentally such a shock, and that my wish had at length been
fulfilled. But, while making this remark, the appearance which my body
presented to myself was that of a number of separate pieces. The arms,
for example, were detached from the trunk, and seemed suspended in the
air. In fact, memory and the power of reasoning appeared to be complete
long before the optic nerve was restored to healthy action. But what
I wish chiefly to dwell upon here is, the absolute painlessness of the
shock; and there cannot be a doubt that, to a person struck dead by
lightning, the passage from life to death occurs without consciousness
being in the least degree implicated. It is an abrupt stoppage of
sensation, unaccompanied by a pang.”
Occasionally branched markings are found on the bodies of those struck
by lightning, and these are often taken to be photographic impressions
of trees under which the persons may have been standing at the time of
the flash. The markings however are nothing of the kind, but are merely
physiological effects due to the passage of the discharge.
During a thunderstorm it is safer to be in the house than out in the
open. It is probable that draughts are a source of some danger, and
the windows and doors of the room ought to be shut. Animals are more
liable to be struck by lightning than men, and a shed containing horses
or cows is a dangerous place in which to take shelter; in fact it is
better to remain in the open. If one is caught in a storm while out of
reach of a house or other building free from draughts and containing
no animals, the safest plan is to lie down, not minding the rain.
Umbrellas are distinctly dangerous, and never should be used during a
storm. Wire fences, hedges, and still or running water should be given
a wide berth, and it is safer to be alone than in company with a crowd
of people. It is extremely foolish to take shelter under an isolated
tree, for such trees are very liable to be struck. Isolated beech trees
appear to have considerable immunity from lightning, but any tree
standing alone should be avoided, the oak being particularly dangerous.
On the other hand, a fairly thick wood is comparatively safe, and
failing a house, should be chosen before all other places of refuge.
Horses are liable to be struck, and if a storm comes on while one is
out driving it is safer to keep quite clear of the animals.
When a Wimshurst machine has been in action for a little time a
peculiar odour is noticed. This is due to the formation of a modified
and chemically more active form of oxygen, called _ozone_, the
name being derived from the Greek _ozein_, “to smell.” Ozone has
very invigorating effects when breathed, and it is also a powerful
germicide, capable of killing the germs which give rise to contagious
diseases. During a thunderstorm ozone is produced in large quantities
by the electric discharges, and thus the air receives as it were a new
lease of life, and we feel the refreshing effects when the storm is
over. We shall speak again of ozone in Chapter XXV.
Thunder probably is caused by the heating and sudden expansion of
the air in the path of the discharge, which creates a partial vacuum
into which the surrounding air rushes violently. Light travels at the
rate of 186,000 miles per second, and therefore the flash reaches us
practically instantaneously; but sound travels at the rate of only
about 1115 feet per second, so that the thunder takes an appreciable
time to reach us, and the farther away the discharge the greater the
interval between the flash and the thunder. Thus by multiplying the
number of seconds which elapse between the flash and the thunder by
1115, we may calculate roughly the distance in feet of the discharge. A
lightning flash may be several miles in length, the greatest recorded
length being about ten miles. The sounds produced at different points
along its path reach us at different times, producing the familiar
sharp rattle, and the following rolling and rumbling is produced by the
echoes from other clouds. The noise of a thunder-clap is so tremendous
that it seems as though the sound would be heard far and wide, but
the greatest distance at which thunder has been heard is about fifteen
miles. In this respect it is interesting to compare the loudest
thunder-clap we ever heard with the noise of the famous eruption of
Krakatoa, in 1883, which was heard at the enormous distance of nearly
three thousand miles.
When Franklin had demonstrated the nature of lightning, he began to
consider the possibility of protecting buildings from the disastrous
effects of the lightning stroke. At that time the amount of damage
caused by lightning was very great. Cathedrals, churches, public
buildings, and in fact all tall edifices were in danger every time
a severe thunderstorm took place in their neighbourhood, for there
was absolutely nothing to prevent their destruction if the lightning
chanced to strike them. Ships at sea, too, were damaged very frequently
by lightning, and often some of the crew were killed or disabled.
To-day, thanks to the lightning conductor, it is an unusual occurrence
for ships or large buildings to be damaged by lightning. The lightning
strikes them as before, but in the great majority of cases it is led
away harmlessly to earth.
Franklin was the first to suggest the possibility of protecting
buildings by means of a rod of some conducting material terminating in
a point at the highest part of the building, and leading down, outside
the building, into the earth. Lightning conductors at the present
day are similar to Franklin’s rod, but many improvements have been
made from time to time as our knowledge of the nature and action of
the lightning discharge has increased. A modern lightning conductor
generally consists of one or more pointed rods fixed to the highest
parts of the building, and connected to a cable running directly to
earth. This cable is kept as straight as possible, because turns
and bends offer a very high resistance to the rapidly oscillating
discharge; and it is connected to large copper plates buried in
permanently moist ground or in water, or to water or gas mains. Copper
is generally used for the cable, but iron also may be employed. In
any case, the cable must be of sufficient thickness to prevent the
possibility of its being deflagrated by the discharge. In ships the
arrangements are similar, except that the cable is connected to the
copper sheathing of the bottom.
The fixing of lightning conductors must be carried out with great care,
for an improperly fixed conductor is not only useless, but may be a
source of actual danger. Lightning flashes vary greatly in character,
and while a carefully erected lightning conductor is capable of dealing
with most of them, there are unfortunately certain kinds of discharge
with which it now and then is unable to deal. The only absolutely
certain way of protecting a building is to surround it completely by a
sort of cage of metal, but except for buildings in which explosives are
stored this plan is usually impracticable.
The electricity of the atmosphere manifests itself in other forms
beside the lightning. The most remarkable of these manifestations is
the beautiful phenomenon known in the Northern Hemisphere as the Aurora
Borealis, and in the Southern Hemisphere as the Aurora Australis.
Aurora means the morning hour or dawn, and the phenomenon is so called
from its resemblance to the dawn of day. The aurora is seen in its full
glory only in high latitudes, and it is quite unknown at the equator.
It assumes various forms, sometimes appearing as an arch of light with
rapidly moving streamers of different colours, and sometimes taking
the form of a luminous curtain extending across the sky. The light
of the aurora is never very strong, and as a rule stars can be seen
through it. Auroras are sometimes accompanied by rustling or crackling
sounds, but the sounds are always extremely faint. Some authorities
assert that these sounds do not exist, and that they are the result
of imagination, but other equally reliable observers have heard the
sounds quite plainly on several occasions. Probably the explanation of
this confliction of evidence is that the great majority of auroras are
silent, so that an observer might witness many of them without hearing
any sounds. The height at which auroras occur is a disputed point, and
one which it is difficult to determine accurately; but most observers
agree that it is generally from 60 to 125 miles above the Earth’s
surface.
There is little doubt that the aurora is caused by the passage of
electric discharges through the higher regions of the atmosphere,
where the air is so rarefied as to act as a partial conductor; and
its effects can be imitated in some degree by passing powerful
discharges through tubes from which the air has been exhausted
to a partial vacuum. Auroral displays are usually accompanied by
magnetic disturbances, which sometimes completely upset telegraphic
communication. Auroras and magnetic storms appear to be connected in
some way with solar disturbances, for they are frequently simultaneous
with an unusual number of sunspots, and all three run in cycles of
about eleven and a half years.