Warren Wilson College

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.


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