In the case of a mile-long flash it has been estimated that 3,516,480 De
la Rue cells, q. v., would be required for the development of the
potential, giving the flash over three and one-half millions of volts.
But as it is uncertain how far the discharge is helped on its course by
the rain drops this estimate may be too high.
There are two general types of flash. The so-called zigzag flash
resembles the spark of an electric machine, and is undoubtedly due to
the disruptive discharge from cloud to earth. Sheet lightning has no
shape, simply is a sudden glow, and from examination of the spectrum
appears to be brush discharges (see Discharge, Brush) between clouds.
Heat lightning is attributed to flashes below the horizon whose light
only is seen by us. Globe or ball lightning takes the form of globes of
fire, sometimes visible for ten seconds, descending from the clouds. On
reaching the earth they sometimes rebound, and sometimes explode with a
loud detonation. No adequate explanation has been found for them.
The flash does not exceed one-millionth of a second in duration; its
absolute light is believed to be comparable to that of the sun, but its
brief duration makes its total light far less than that of the sun for
any period of time.
If the disruptive discharge passes through a living animal it is often
fatal. As it reaches the earth it often has power enough to fuse sand,
producing fulgurites, q. v. (See also Back Shock or Stroke of
Lightning.)
Volcanic lightning, which accompanies the eruptions of volcanoes, is
attributed to friction of the volcanic dust and to vapor condensation.
[Transcriber's note: The origin of lightning is still (2008) not fully
understood, but is thought to relate to charge separation in the
vertical motion of water droplets and ice crystals in cloud updrafts. A
lightning bolt carries a current of 40,000 to 120,000 amperes, and
transfers a charge of about five coulombs. Nearby air is heated to about
10,000 °C (18,000 °F), almost twice the temperature of the Sun’s
surface.]
Lightning Arrester.
An apparatus for use with electric lines to carry off to earth any
lightning discharge such lines may pick up. Such discharge would imperil
life as well as property in telegraph offices and the like.
Arresters are generally constructed on the following lines. The line
wires have connected to them a plate with teeth; a second similar plate
is placed near this with its teeth opposite to those of the first plate
and nearly touching it. The second plate is connected by a low
resistance conductor to ground. Any lightning discharge is apt to jump
across the interval, of a small fraction of an inch, between the
oppositely placed points and go to earth.
Another type consists of two plates, placed face to face, and pressing
between them a piece of paper or mica. The lightning is supposed to
perforate this and go to earth. One plate is connected to the line, the
other one is grounded.
The lightning arrester is placed near the end of the line before it
reaches any instrument. (See Alternative Paths.)