To give a few random examples: in 1902 lightning destroyed an upper section of the Eiffel Tower; in 1971 lightning struck a wing fuel tank on LANSA Flight 508 in Peru, killing 91 people2; in 1994 lightning struck some ground fuel tanks in Egypt, killing 469 people; and 30 cows died sheltering under a tree in 2004 in Denmark.
2Astonishingly, there was one survivor, a 17-year-old German high school student who fell two miles, still strapped to her seat, into the jungle. Despite a broken collar bone and an eye injury, she managed to follow a stream, ultimately finding a loggers’ camp. “Wings of Hope” (2000) is a TV documentary about the ordeal.
24 1 Home Electrostatics
There are many different classifications of lightning—ball lightning, sheet lightning, and so on—but the commonality is the separation of (+) and ( ) charges, which, if they get too large, may “discharge” suddenly. (The same happens, but in a controlled way, with an automobile spark plug.) Since gases and liquids are FLUIDS what we mean by “discharge” is that, if there is a high concentration of oppositely charged regions, individual charged atoms and molecules are free to move about in what is quite often an explosive release. Another example of a discharge would be in the gas of a camera flash tube. In SOLIDS there is a fixed framework of atoms constraining any possible discharges.
In Figs.1.26and1.27we have assumed that the charges in a cloud have become separated, possibly due to violent updrafts in hot weather or atmospheric instabilities.
Let’s say that this “polarization” of charges in a storm cloud is vertical—which makes sense for updrafts—with the lower part of the cloud positive and the upper part negative.
Benjamin Franklin, following a 1742 German device that he had read about, set up in his house in Philadelphia a pointed conductor sticking above his chimney, with the lower end attached to a bell inside the house. He wrote: “. . . in 1752 I erected an iron rod. . .about nine feet above the chimney. . .down into my house
. . .and. . .the bells rang when there was a[thunder]cloud over the house.”
He reasoned that the top of his conductor would be of one polarity (see Fig.1.26) and the bottom end of the opposite polarity. The migration of the ( )s to the top of the rod leaves the right bell (+) and the left bell ( ).
brass ball, attracted first to one side, then the other
−ground
− +
+
−
−
+ + + + +
Fig. 1.26 Ben Franklin’s thunder cloud warning bells, Philadelphia, 1752
A second bell was attached to ground. This bell would be relatively ( ) because the local ( ) charges in the earth try to get as close as possible to the (+) on the first bell.
So far nothing happens, but if a little metal ball (Franklin’s was brass)—
suspended by a silk or other insulating thread—is to hang between the two bells, then the ball will at first be attracted to one bell and, immediately after contact (thus acquiring a (+) charge), be repelled. Consequently, it will then be attracted over to the opposite bell—and so on, back and forth—ringing wildly!
Franklin’s lightning bells are not a safe “kitchen” experiment for us to try.
A Swedish experimenter had actually been killed from lightning discharges in the 1750s; fortunately, Ben Franklin, experimenting during storms, avoided such a fate.
Fig. 1.27 Charges at the bottom of the cloud attract opposite charges from the ground
26 1 Home Electrostatics
Also, as mentioned in the introduction, it is not safe to shelter under a tree during a storm. While the discharge, or spark, through the air can take quite jagged paths—
and more than one—some possible paths have been sketched in Fig.1.27.
In this figure negative charges are attracted upwards from tall objects towards the lower (+) of the storm cloud. The discharge, if it occurs, is across the air as well as down multiple pathways inside the tree and down from the tree. One possible path could be via the body of the person standing under a branch.
One of the safest places to be in a lightning storm is inside a car. If the metal of the car should become charged, we know that the charges will get as far away from each other as possible—i.e., to the outside of the metal. This is an example of a “Faraday cage”—named after Michael Faraday (see p. 106) who built such a cage to demonstrate that charges reside on the outside of conductors. (The wetness of the tires of the car have no bearing on this, although they do provide a possible discharge path.) We will also see that when the charges suddenly move, the flow is still on the outside due to something called the “skin effect” (described in Chap.2, p. 74).
Thus passengers in metallic cars, planes, and so on are protected from lightning from two distinct points of view.
Finally, although planes are hit many times a year by lightning, the last crash was 40 years ago, because a fuel tank was insufficiently protected. Since that time, aeronautical designers have been extremely careful about the shielding of fuel tanks.
Fig. 1.28 Shows an aircraft flying in the vicinity of storm clouds, with the discharge between the two clouds flowing along the outside of the metal fuselage