By drawing lightning from storm clouds, Franklin’s dangerous kite experiment conclusively demonstrated that lightning was a form of electricity. The experiment also offered further proof of his single-substance theory of electricity and showed that this fluidlike static energy could be passed from one object to another.
Benjamin Franklin’s kite experiment
Benjamin Franklin found that lightning was a form of electricity, leading to his invention of the lightning rod.
As early as 1749, Franklin had suspected that lightning was an electrical discharge from storm clouds. In a letter to Collinson, he observed that lightning and electricity shared similarities in color, crookedness of motion, and crackling sounds. If lightning is electricity, Franklin wondered, then how did the clouds obtain this electrical static? He conjectured that salt particles found in oceans rub against water to produce an electrical charge on the surface of the seas. Through evaporation, he further speculated, this charge rises to the clouds, which during certain types of encounters release this charge as lightning.
Franklin also observed that tall objects, such as steeples, trees, or ship masts, can trigger the release of electrical energy from clouds. To protect such structures from fires caused by lightning strikes, he recommended 10-foot-long “upright rods of iron made sharp as a needle” (lightning rods). Extended from the peaks of high structures, these devices would preemptively attract “electrical fire” from the clouds. Although his report on the kite experiment in the October 19, 1752, edition of The Pennsylvania Gazette states that a test of his lightning-rod theory succeeded in Philadelphia, it remains uncertain whether he personally ever conducted such a trial by this date.
On the other hand, it is virtually certain that with the assistance of his twenty-one-year-old son William in June, 1752, Franklin did indeed conduct a kite test of his theory of the electrical nature of lightning. In a letter to Collinson dated October 19, 1752, as well as in the Gazette article published that same month, Franklin reported that during a thunderstorm he flew a kite made of a silk handkerchief tied across cedarwood crosspieces. The kite had a tail, and a foot of wire extended as an antenna from its top end. The lower end of the twine descending from the kite was tied with an insulating silk ribbon, from which an iron key was suspended away from his hand by a cotton string. To keep the silk ribbon and the key dry during the storm, Franklin stood inside a doorway while flying the kite. When he passed his other hand over the key, a spark leaped from the key toward his knuckle. This transfer of energy proved that lightning was electrical in nature.
Franklin’s use of a pointed-tip conductor in the kite experiment also advanced the case for his single-substance (or one-fluid) theory of electricity.
The kite experiment showed, moreover, that the common substance of lightning’s electric fluid can be passed
Benjamin Franklin’s dramatic kite experiment became an instant legend. A recurrent subject for paintings and print illustrations over the centuries, the kite test is now ingrained in the world’s cultural memory of Franklin. As early as 1767, Joseph Priestley, in his History and Present State of Electricity,
From the standpoint of the history of physics, however, the result of Franklin’s kite experiment with lightning is not considered as significant as Priestley thought. Franklin did not know that in France, a month before the Philadelphia kite test, experimenter Thomas-François Dalibard
The kite experiment in Philadelphia was, finally, most significant for the evidence it provided in support of two of Franklin’s major contributions to the study of physics: his single-substance theory of electricity and his related invention of the pointed-tip lightning rod, still used today. In 1754, two years after Franklin’s kite project, the Royal Society of London
In America it was the pointed-tip lightning rod on the kite, more than his single-substance theory, that elevated Franklin’s fame. Franklin’s lightning-rod design, which would become the worldwide standard, was indeed more effective than the European blunt-tipped model. However, its effectiveness when employed to protect buildings from electrical fires resulting from lightning strikes was not its only value for Americans. The strategic foot of wire “made sharp as a needle” and extended skyward from the top of the kite’s wooden crosspieces was the prototype for the lightning rod that would become not only a practical invention but also an “electrifying” cultural symbol for eighteenth century America. Elated because this colonial device was superior to the European version, Americans proudly celebrated the design of Franklin’s lightning rod as a symbol of their new nation’s ingenuity and independence.
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Cohen, I. Bernard. Benjamin Franklin’s Science. Cambridge, Mass.: Harvard University Press, 1990. Cohen’s work, especially sensitive to Franklin’s Newtonian heritage, also discusses other kite experiments. -
Hankins, Thomas L. Science and the Enlightenment. New York: Cambridge University Press, 1985. Chapter 3 details the eighteenth century background of Franklin’s work with electricity. -
Schiffer, Michael Brian. Draw the Lightning Down: Benjamin Franklin and Electrical Technology in the Age of the Enlightenment. Berkeley: University of California Press, 2003. Presents Franklin as a leading figure in eighteenth century electrical research and especially highlights his theory of electricity and invention of the lightning rod. -
Tanford, Charles. Franklin Stilled the Waves. Durham, N.C.: Duke University Press, 1989. Chapters 2 and 3 feature Franklin’s work on electricity in the context of his other scientific experiments.
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Henry Cavendish; Benjamin Franklin; Luigi Galvani; Joseph Priestley; Alessandro Volta.