Throughout much of recorded history,
lightning was feared and often seen as a form of punishment
(though also sometimes associated with fertility - eg. birth of
twins).
“symbols based
on the concept of fire are among the oldest pictorial
representations of lightning.” (see
Ref. (1))
“In many
ethnological representations from prehistoric times lightning is
depicted …. as a stone falling from heaven or a stone axe hurled
from the skies.” (Ref(1)).
Stones falling from the skies seems to be a very widespread
association.
“French peasants carry a “
When I taught this course in Spring 2011 I wasn't able to
find much additional information. This year was
different. Wiktionnaire (the
French version of Wiktionary) defines pierre de tonnerre as:
"Boule de Marcassite" (ball of marcasite) and "Hache polie"
(polished axe) and gives a couple of synonyms: "boule de
tonnerre" and "pierre de foudre" (lightning stone).
Wiktionnaire goes on to explain that a thunder stone is
"supposee naitre de la foudre frappant le sol" (thought to be
created by lightning striking the soil). The
balls of marcasite are frequently found in the chalky soils of
the Champagne region.
Marcasite is a mineral and is sometimes called white iron pyrite. Here are a couple of pictures.
Boule de tonnerre (source)
A marcasite geode cut in half and polished (source)
A ball of marcasite has an unusual appearance and must be heavy because iron pyrite is dense (about 4.9 g/cm3). But I was beginning to wonder how could iron pyrite be produced by lightning striking chalky soil.
Here's an image of a polished neolithic axe (source)
Clearly this wasn't produced by lightning. But an object like this would qualify as a "pierre de tonnerre" and was thought to offer protection from lightning.
Some addtional research led to a site that mentioned an "oeuf de tonnerre" (if you click on the link you'll find the picture below and many more beautiful examples like it, oeuf by the way is the French word for egg). These "thunder eggs" are produced in cooling lava as best I can tell.
The site explains the origin of the term thunderegg:
Selon la légende Amérindienne ,lorsque les esprits de la tempête qui vivaient sur les sommets enneigés du Wy'east (mont Hood) et le Mont jefferson dans l'orégon, se mettaient en colère les uns contres les autres, ils se lançaient des boulets de rochers, les dieux trouvaient ces armes dans les nids des oiseaux tonnerre (thunderbirds) d'ou la dénomination Thundereggs.
A rough translation: According to American Indian legend, when the storm spirits that lived on the snow covered summits of Wy'east (Mt. Hood) and Mt. Jefferson (in Oregon) would throw balls of rock at each other when they became angry. The gods founds these weapons in the nests of thunderbirds hence the term thunderegg.
A final site explains that a pierre de tonnerre is often just a fragment of a meteorite.
So after a somewhat time consuming, but interesting, detour I concluded that none of these objects was actually produced by lightning. Rather, at one time, becuase of their unusual appearance and weight were thought to be produced by lightning or, in the case of a meteorite, observed to fall from the sky. They were something of value because the belief was that lightning would never strike the same location twice and thus either carrying one of these stones or putting it on the threshold of a home would protect the bearer or the structure from lightning.
There were many people in the American
Colonies and Europe that were interested in and actively
studying electricity at the time.
Electrostatic
machines (friction machines) were in wide
use. Peter Collinson – unpaid
Franklin believed (correctly) that rubbing two materials
together did not create electricity. Rather, the
rubbing somehow or another "grabbed onto" and separated
charges that already were part of the neutral
materials. Material 1 might "tear" electrons from
material 2. Material 1 would become negatively
charged and material 2 would be left positively charged.
Priestley explains this more clearly and succintly:
“Dr.
Franklin
had
discovered … that the electric matter was not created but
collected by friction, from the neighbouring non electric
bodies.” Priestley was the first
historian of electrical science. His
“History and Present State of Electricity” appeared in
You'll find a short
description (and some explanation) of the experiment
on Wikipedia. A portion is reproduced below
A popular but misleading demonstration with a Leyden jar involves taking one apart after it has been charged and showing that the charge is stored on the dielectric not the plates. The first documented instance of this demonstration is in a 1749 letter by Benjamin Franklin. Franklin designed a "dissectible" leyden jar, shown below, which was widely used in demonstrations.
The jar in the demonstration is constructed out of a glass cup nested between two fairly snugly fitting metal cups. When the jar is charged with a high voltage and carefully dismantled, it is discovered that all the parts may be freely handled without discharging the jar. If the pieces are re-assembled, a large spark may still be obtained.
When not properly explained, this demonstration promotes the myth that capacitors store their charge inside their dielectric. This erroneous theory, due to Franklin, was taught throughout the 1800s, and is still sometimes encountered. However this phenomenon is a special effect caused by the high voltage on the Leyden jar. In the dissectible Leyden jar, charge is transferred to the surface of the glass cup by corona discharge when the jar is disassembled; this is the source of the residual charge after the jar is reassembled. Handling the cup while disassembled does not provide enough contact to remove all the surface charge.
The demonstration was performed
in class. A Van de Graaff generator (invented in 1929)
was used to charge the Leyden jar. A Van de Graaff
generator is an example of a friction machine and it is worth
taking a moment to understand how they work. A photo is
show below at left and a sketch at right.
 |
 |
The lower roller (1) is made of
plexiglas. Friction between the belt and the plexiglas
roller causes the roller to become positively charged (the
inside surface of the belt is negatively charged). We
know this from the Triboelectric Series shown below.
A strong electric field develops between the positively charged plexiglas roller and the comb of metal points connected to ground. The electric field is strong enough to ionize the air and corona discharge "sprays" electrons onto the belt. This charge is then carried upward toward the top of the generator (3). The upper metal comb draws off negative charge from the belt and it moves to the surface of the metal dome.
Friction between the belt and
the upper roller (made of polyethylene) causes the upper
roller to become negatively charged. Corona discharge
between the comb and the negatively charged roller adds
additional negative charge to the dome.
If this explanation is correct
(and I'm not entirely sure it is) we should find that the dome
in this case is negatively charged. Later in the course
we'll use an electric field mill to determine whether that is
really the case.
Thus, it is not very predictable, and only broad
generalizations can be made. Amber,
for example, can acquire an electric charge by contact and
separation (or friction) with a material like wool.
This property, first recorded by Thales
of Miletus, suggested the word "electricity" (from William Gilbert's
initial coinage, "electra"), from the Greek word for amber, ēlektron.
Other examples of materials that can acquire a significant
charge when rubbed together include glass
rubbed with silk, and hard rubber rubbed with fur."
And here is list many materials and the charge they acquire (from the same article in Wikipedia). This was on a handout distributed in class.
The Triboelectric Series
| Positively charged (most charging at the top of the list) |
No charge |
Negatively charged (most charging at the top of the list |
| polyurethane foam Hair, oily skin nylon, dry skin glass acrylic, lucite leather rabbit's fur quartz mica lead cat's fur silk aluminum paper (small positive charge) cotton |
wool steel |
ebonite silicone rubber teflon silicon vinyl (PVC) polypropylene polyethylene (like Scotch tape) plastic wrap orlon sytrene (Styrofoam - polystyrene foam) polyester synthetic rubber acetate, rahyon gold, platinum brass, silver sulfur nickel, copper hard rubber resins rubber balloon polystyrene sealing wax amber wood (small negative charge) |
This is what was used to determine the charging of the plexiglas and polyethylene rollers in the
Van de Graaff generator
This was demonstrated using a Van de
Graaff generator. We first position a grounded metal
rod with a rounded tip a few centimeters from the top of the
generator. Periodically, once sufficient charge builds
up on the dome of the generator, an audible visible spark
(about 3 inches long) will jump to the tip of the ground
rod.
 |
 |
If a pointed, grounded rod is brought
to within about 20 centimeters of the Van de Graaff, the
sparking to the grounded round ball stops. The pointed
rod is drawing off electricity from the generator before
sufficient charge is able to build up and spark across to
the grounded ball.
The terms drawing off or
throwing off electricity simply refer to whether current
is flowing to or from the pointed rod.
Franklin originally thought a lightning rod would
work in this way.
"To determine the question, whether
the clouds that contain lightning are electrified or not, I
would propose an experiment to be tried where it may be done
conveniently. On the top of
some high tower or steeple, place a kind of sentry-box big
enough to contain a man and an electrical stand. From the middle of the stand let
an iron rod rise and pass bending out of the door, and then
upright 20 or 30 feet, pointed very sharp at the end. If the electrical stand be kept
clean and dry a man standing on it when such clouds are
passing low, might be electrified and afford sparks, the rod
drawing fire to him from a cloud. If
any danger to the man should be apprehended (though I think
there would be none) let him stand on the floor of his box,
and now and then bring near to the rod the loop of a wire
that has one end fastened to the leads, he holding it by a
wax handle; so the sparks, if the rod is electrified, will
strike from the rod to the wire, and not affect him.”
The experiment was performed for the
first time on May 10, 1752 in Marly-la-Ville (near Paris) by
a retired dragoon name Coiffier (Thomas Francois Dalibard, a
naturalist, was absent). Dalibard read an account of
the experiment to the French Academie des Sciences on May
13, 1752. You can read a short description of the experiment
(in French) on the Commune
de
Marly La Ville website.
The experiment was widely repeated
LeMonnier held a 5 m wooden pole with iron wire windings while standing on pitchcake. Sparks were seen coming from his hands and face.
“There is
something however in the experiments of points, sending
off, or drawing on, the electrical fire, which has not
been fully explained, and which I intend to supply in my
next. For the doctrine of points is very curious, and the
effect of them truly wonderful; and, from what I have
observed on experiments, I am of opinion,
that houses, ships, and even towns and churches may be
effectually secured from the stroke of lightning by their
means; for if, instead of the round balls of wood or
metal, which are commonly placed on the tops of the
weather-cocks, vanes or spindles of churches, spires, or
masts, there should be put a rod of ion 8 or 10 feet in
length, sharpen’d gradually to a point like a needle, and
gilt to prevent rusting, or divided into
a number of points, which would be better – the electrical
fire, would, I think, be drawn out of a cloud silently,
before it could come near enough to strike; only a light
would be seen at this point, like the sailors corpusante.”
and my favorite quotation:
"It has pleased God in his goodness to mankind, at
length to discover to them the means of securing their
habitations and other buildings from mischief by
thunder and lightning ..."
Here Franklin was anticipating and seeking
to counter opposition from religious authorities
(lightning was considered by many to be a form of
divine retribution).
