The earliest men must have thought that the vivid lightning flash and the
roaring voice of thunder that followed it were the anger of the gods, especially
when they sometimes saw other men struck and killed by lightning. In fact,
lightning is a gigantic electric spark. If wires connected to the two terminals
of an electric battery are brought very close together, a little spark will jump
between them. Lightning flashes are enormous electric sparks jumping from one
part of a thundercloud to another, or from the cloud to the ground.
may be up to eight kilometers long, and to jump such distances they need a
voltage or electrical pressure several hundred times as great as that of the
electricity supplied to our houses. Scientists do not fully understand how this
great electrical pressure is built up in a thundercloud. But when sufficiently
charged, the thundercloud will emit a lightning flash which takes place in the
form of a long spark which follows the path of least electrical resistance. It
often appears as a zigzag line called forked or chain lightning, which leaps
from the cloud to the ground in less than one-tenth of a second. Where it
strikes the ground, solid rock may be melted by the huge power of the electrical
discharge. It is this kind of lightning that sometimes kills people. However,
there is not much risk of being struck by lightning unless you are at or near
the highest point in the neighborhood or are sheltered under a tall tree
standing by itself. Indoors there is little risk.
Sheet lightning is a flash
from one part of a thundercloud to another or from one cloud to the next. Summer
lightning or heat lightning is the reflection in the clouds of flashes a long
way off. The thunder that follows a lightning flash is caused by the sudden
expansion of the air, since nay electric spark causes a very large rise in
temperature in the nearby air.
A building is not likely to be struck if
protected by a lightning conductor. This device was invented in 1752 by Benjamin
Franklin. It consists of a spiked metal rod fixed above the highest point of the
building and connected by a copper strip to rods or wires buried in the ground.
its principal function is to prevent lightning, or at any rate reduce its
severity by discharging the cloud harmlessly. The spikes direct a stream of
neutralizing charges at the cloud above, disarming it and rendering it harmless.
If lightning does occur, the discharge will not be so strong and the metal of
the conductor is an easy path to earth. Thus the electricity passes harmlessly
through the conductor instead of tearing through the building.
The sparks in
an electric drill or a motor car engine give out radio waves which cause
interference in radio or television sets. Lightning flashes are gigantic sparks,
and so can cause interference over large distances. This is heard as scratching
and crackling sounds on a radio.
Although lightning does a great deal of
damage throughout the world, it has one good effect. The flash causes the
nitrogen and the oxygen in the air to combine, and when dissolved in the
raindrops, they fall to the earth as natural nitrates. Nitrates are a valuable
fertilizer. This means they help plants and crops to grow.