Physics
Photo of the Week
September
15, 2006
Lightning
Although
this photo was made last May 13, it certainly has been indicative of
early September 2006 weather in Western North Carolina. The photo
was made at at 9:41 pm from the shelter under the eves of my
house. It also was not raining at the camera location.
However, much rain can be seen in the center of the photograph looking
toward Swannanoa, NC. The photograph was made by taking a time
exposure for 15 seconds. The lightning illuminates the clouds and
rain squall.
Lightning was
featured in an earlier Physics Photo of the week. Dan Sockwell's
photograph and excellent account was published October
1, 2004.
In This
article I will attempt to summarize the understanding of
lightning made by Benjamin Franklin in the mid 1700's. The
information about Benjamin Franklin and his lightning experiments is
found in a wonderful biography: Benjamin
Franklin
an American Life by Walter
Isaacson, Simon and Schuster, 2003.
Franklin had
been experimenting with electrostatic phenomena and sparks created by
rubbing glass rods with silk, wool, cat fur, etc. Franklin
contributed a tremendous breakthroughs in the understanding of
electricity: 1) that electrical charges were not created by friction, but collected by friction, and 2) there
are two types of charges he called "plus" and "minus" and that for
every plus charge collected, a minus charge was left behind. The
idea of charge conservation is as physically significant as Newton's
idea of momentum conservation. One of the main tools for
Franklin's experiments with electricity was the simple Leyden jar
capacitor - a simple device that could store electrical charge.
[Capacitors are important components in modern electronic devices and
circuits].
He also
discovered in his home experiments that sharp pointed needles would
slowly discharge a charged metal ball in its vicinity without
generating a spark. A smooth object in the vicinity of the
charged ball, on the otherhand, would not slowly discharge the charged
ball, but induced a large spark when brought close enough. We
perform similar experiments in the Physics Labs at Warren Wilson
College.
The effect of sharp points on the discharging of electrified objects
gave Franklin that this idea could 
perhaps minimize
the dangers wrougt by lightning. But first he had to demonstrate that
lightning from the sky is the same but larger electrical phenomena that
was generated with glass rods, silk, and stored in a Leyden jar capacitor
(drawing at left from Wikipedia). He proceded to plan some
dangerous experiments
to erect a long pole onto the top of a church steeple (under
construction) and
see if he could generate sparks between the base of the pole and a
grounded loop of wire held by an experiment with a wax insulating
handle. This time the sparks would come from the sky, not by
rubbing materials. As with practically all construction projects,
the steeple construction was delayed and Franklin was impatient for his
scientific results. Since he was also quite skilled with kites,
he
constructed a silk kite that had a sharp wire attached to pick-up
electrical
charges from the sky. He also had a dry shed available in an open
field that would provide shelter while flying the kite in or near a
thunderstorm. Thus he was ready for the experiment when a proper
storm encountered the area. The kite, in addition to the wire
antenna, was flown with a silk string. The wetness of the silk provided enough
electrical conductivity to bring some of the electrical
charges from the sky to the experiments in the shed.
Franklin's
experiment in the shed consisted of tying a metallic key to
the kite string. I believe that the string was tied to a wooden
post
in the shed while the wind was able to maintain the kite in
flight.
After flying the kite for awhile, he noticed that the strands on the
string stiffened, presumably indicating that the string was
electrically charged. He approached the suspended key with
his knuckle and sure-enough, a miniature spark jumped between
the key and his finger - exactly as sparks resulting from charged
objects from rubbing materials - combing hair, stroking cats,
etc. He was also able to charge a Leyden jar with the electricity
from the sky through the kite string. I do not believe that
lightning struck the kite (Franklin noted "small" sparks). The
kite, and antenna attached to it, acted like pointed object and slowly
drew the electricity from the sky to be stored in the Leyden jar.
The "electric
kite" experiment (done "secretly" with his son William)
convinced
Franklin that buildings (church steeples, barns, and houses) could be
protected from lightning by
means of lightning rods: pointed metal rods attached to the roofs and
connected with a conducting wire to the ground. Contrary to much
popular belief, lightning rods provide protection by slowly discharging
the electrically charged clouds above the structure, rather than
attracting a bolt of lightning to strike the rod and not the
house. In the WWC physics demonstration, a student (or professor)
safely holds a pointed thumb tack close to a charged Van de Graaf
generator. With the pointed thumbtack, the experimenter does not
get struck by a large spark because the pointed tack slowly discharges
the generator. Without the tack, the professor receives a rather
large spark...
At Warren Wilson College, lightning rods are installed on the
Chapel. They are inconspicuous pointed copper rods connected to
the ridge of the structure. The rods are connected to heavy
copper cables leading to the ground.
Physics
Photo of the
Week is
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature an interesting phenomena in
the world around us. Students, faculty, and others are invited to
submit digital (or film) photographs for publication and
explanation. Atmospheric phenomena are especially welcome.
Please send any photos to dcollins@warren-wilson.edu.
Click
here to see the Physics Photo
of
the Week Archive.
Observers
are invited to submit
digital photos to:
