In this lecture we'll continue our study of some of the important
historical developments in Atmospheric Electricity. Many of
the significant early contributions came from Benjamin
Franklin. That's what we will concentrate on here.
Additional details can be found in the references listed at the
end of this lecture.
We'll concentrate on three of
Franklin's contributions:
1. The power of
points
One of Franklin's first observations was "the
wonderful
effect of pointed bodies, both in drawing off and throwing
off the electrical fire.”
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.
2. Suggestion and proof that
thunderstorm and laboratory electricity were the same.
Franklin saw many similarities between the electricity used
in his experiments and lightning.
Both produce light, and the colors of light are similar.
Crooked channels. Swift motion. Being conducted by
metals. Crack or noise produced during discharge.
"Subsisting" in water or ice. "Rending" bodies as current
passes through. Killing animals. Melting metals.
Catching inflammable materials on fire. Sulphurous smell.
He wondered whether lightning wasn't just a much larger
scale form of the same phenomenon and proposed the following
experiment (the Sentry box experiment was described in a July
29, 1750 letter)
"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.”
figure above is from Uman's 1987 book "The Lightning Discharge."
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 repeated
for the French king, Louis XV, a short time later.
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.
Franklin never did the
sentry box experiment (he thought the metal rod would need
to be higher and came up with the idea of using a kite)
This figure is also from Uman's 1987 book "The Lightning
Discharge"
The experiment is thought to have been conducted in June,
1752, but the exact date and location were never
recorded. Details of the experiment were
sent to Collinson in a letter dated Oct. 19, 1752.
Other people began to
repeat the experiment using rockets (mortars) and
balloons. In June 1753 de Romas used a kite with a
240 m cord wrapped with violin wire. He produced 20
cm long sparks. Apparently he was later able to
produce 3 m long sparks!
The strength of the electricity was often judged by
simulating the muscles of animals and observing their
reaction.
from Ref (1)
Both the sentry box experiment and the kite
experiment are very dangerous.
If lightning were to strike the metal pole or the kite or
balloon, the person at the bottom would likely be
killed. This did eventually happen
Figure from ref (2) 3. Invention of lightning rods
Franklin came up with
the idea of a lightning rod:
“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 dividedinto
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.”
“I
say,
if
these
things are so, may not the knowledge of this power of
points be of use to mankind, in preserving houses,
churches, ship etc. from the stroke of lightning, by
directing us to fix on the highest parts of those
edifices, upright rods of iron made sharp as a needle, and
gilt to prevent rusting, and from the foot of those rods a
wire down the outside of the bulding into the ground, or
down round one of the shrounds of a ship, and down her
side till it reaches the water?”
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).
Franklin originally
thought (incorrectly) a lightning rod would dissipate
electricity (the pointed tip would draw off electricity
before a discharge could occur). The first strike to
one of Franklin's rods melted the tip of the rod which
surprised him.
Metal (nail) rods were often
linked together as shown below (some fragments of
Franklin's original lightning rods still exist, in one
case inside a building and next to dry wooden
beams). The links tended to rupture.
As problems became apparent Franklin worked to
make improvements. In particular he investigated
the following:
how does the rod work
what material should be used
termination in air
grounding
attachment to structure
height above the structure
area protected by the rod
Considerable opposition to
the use of lightning rods in Europe. They didn't
believe that it would dissipate the electricity
(correct). Franklin argued that even if not, the
lightning rod and wire to ground would safely carry the
lightning current around and thereby protect the
structure.
"Lightning had been regarded as a divine
expression, a manifestation against which
there could be no possible protection, except
prayer and the ringing of church bells.
Such bells cast in mediaeval times often bore the legend "Fulgura frango"
("I break up the lightning"). With
the passage of time, however, it was realized that bell
ringing during a storm was a very hazardous remedy,
especially for the ringer on the ropes becuase so many
were killed by the very stroke they attempted to
disperse." In 33 years of lightning strokes on 386
church steeples 103 bell-ringers were killed. (Ref (2))
A showdown took place in the Piazza in Siena
Italy in Spring 1777. One side doubted the
electrical nature of lightning and the efficacy of
lightning rods. A second, more progressive side, had
ordered a lightning rods to be installed on the cathedral
and the tower of City Hall (facing the plaza where the
famous Palio
is run).
"On the afternoon of 18 April clouds began to form,
distant thunder was heard, and the Siennese began moving
to their Piazza with all eyes focused on the lightning rod
tip. At about five o'clock - lightning struck.
A ball of fire, accompanied by sparks, smoke, and an odor
of sulphur ran the full length of the tower and
disappeared into the ground leaving the tower unharmed."
(source
of the image above)
Lightning rods were quickly adopted throughout
Italy (and also in other Catholic countries because they
were approved by The Pope)
A lightning house, a common demonstration of the
efficacy of lightning rods. The small square in
the side of the house is filled with gunpowder.
When a spark is delivered to Point V it will travel
down Conductor S, spark across the Gap Q-O and ignite
the gunpowder. If a metal connection is made
between Q and O, the current will flow through a metal
conductor all the way to the ground. There won't
be any sparking and the gunpowder won't be ignited.
It is hard to appreciate the acclaim that Franklin's ideas
and experiments in electricity brought to him in Europe
(though he did also have some enemies)
May 1752 Congratulations from the King of France
July 1753 Master of Arts from Harvard University
Sept. 1753 Master of Arts from Yale University
Nov. 1753 Copeley Gold Medal, Royal Society, London
April 1756 Fellow (w/o fee) of the Royal Society
Feb. 1759 University of St. Andrews Scotland Doctor of Civil
and Canon Laws
April 1762 Oxford Doctor of Civil Laws
Note the Franklin chimes (center left) and a grounded
lightning rod (window)
A Franklin chimes was demonstrated in class. The
photograph below shows the apparatus that was used.
It consists of two bells. The left bell is connected
to ground. The right bell was insulated from ground on a
teflon support. A pointed rod extended upward from the
right bell. The point at the end of the rod was placed 3
or 4 inches from the top of the Van de Graaff generator.
A small metal ball (actually a piece of rod) was suspended on
an insulating string midway between the two bells.
When the Van de Graaff was turned on the metal ball began to
swing between the two bells. The operation is explained
in the following series of pictures.
The pointed rod draws charge from the Van de
Graaff generator and charges the right bell. The metal
ball between the two bells is initially uncharged.
The induced charges shown in the figure keep the electric
field inside the conductor zero.
The metal ball is attracted to the right bell. Contact
with the bell neutralizes the negative charge on the
ball. The ball is then repelled by the positive charge
on the bell.
Once the ball touches the grounded bell, the positive charge
flows to ground.
The ball is again uncharged. The whole process repeats
itself. The overall effect of the swinging ball is to
transport charge from the right bell to ground.
The Franklin chimes demonstration was followed by a
demonstration of Volta's Hailstorm.
The Volta Hailstorm apparatus consisted of two square metal
plates mounted on the bottom and top of a clear plastic or
glass cylinder. Small round balls made of
aluminum foil were inside. The apparatus was placed on
top of a Van de Graaff generator. The top metal
plate was connected to ground.
When the generator is turned on, the foil balls which are in
electrical contact with the generator dome acquire some charge
(assumed to be positive).
The balls are repelled by the bottom electrode and travel up
to the top metal disk (left figure above). They transfer
their charge to the top disk and then fall back to the bottom
disk (right figure). The positive charge in the top
plate then flows to ground. The foil balls
again acquire charge from the bottom disk and the whole
process repeats itself.
The apparatus will sometimes operate even when the top plate
is not directly connected to ground. The charge on the
top disk bleeds off off the corners of the top plate and
through the pieces of wire connected to the plate (the pointed
pieces of wire are "throwing off" the electrical charge; the
charge eventually travels to ground and completes the
circuit). The foil balls again acquire charge from the
bottom disk and the whole process repeats itself.
The links above will take you to an online e-journal copy
of the article available from the University of Arizona
Library; it may not be available to you if you use an
off-campus computer.