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The kite experiment was a scientific experiment which was proposed and may have been conducted by Benjamin Franklin with the assistance of his son William Franklin. The experiment's purpose was to uncover the unknown facts about the nature of lightning and electricity.
Contents
Background
In 1750, the electrical nature of lightning was the subject of public discussion in France, with a dissertation of Denis Barbaret receiving a prize in Bordeaux; Barbaret proposed a cause in line with the triboelectric effect. The physicist Jacques de Romas also wrote a memoir that year with similar ideas. Franklin had listed a dozen analogies between lightning and electricity in his notebooks at the end of 1749. Speculations of Jean-Antoine Nollet had led the issue being posed as a prize question at Bordeaux in 1749. De Romas later defended his own electrical kite proposal as independent of Franklin's.
Lightning rod experiments
In 1752, Franklin proposed an experiment with conductive rods to attract lightning to a Leyden jar, an early form of capacitor. Such an experiment was carried out in May 1752 at Marly-la-Ville in northern France by Thomas-François Dalibard. An attempt to replicate the experiment killed Georg Wilhelm Richmann in Saint Petersburg in August 1753; he was thought to be the victim of ball lightning. Franklin himself is said to have conducted the experiment in June 1752, supposedly on the top of the spire on Christ Church in Philadelphia.
Franklin's Kite Experiment
Franklin's kite experiment was performed in Philadelphia in June 1752, according to the account by Priestly. Franklin described the experiment in the Pennsylvania Gazette in October 19, 1752, without mentioning that he himself had performed it. This account was read to the Royal Society on December 21 and printed as such in the Philosophical Transactions. A more complete account of Franklin's experiment was given by Joseph Priestley in 1767, who presumably learned the details directly from Franklin, who was in London at the time Priestley wrote the book.
Franklin realized the dangers of using conductive rods and instead used the conductivity of a wet hemp string attached to a kite. This allowed him to stay on the ground and the kite was less likely to electrocute him. According to the account, Franklin kept the string of the kite dry at his sons's end to insulate him while he was allowed to get wet in the rain to provide conductivity. A house key belonging to Benjamin Loxley was attached to the hemp string and connected to a Leyden jar; a silk string was attached to this, held by Franklin's son William who flew the kite (while standing inside a doorway to keep himself and the silk string dry). Franklin assumed that the Leyden jar would accumulate electricity from the lightning. The kite was not struck by visible lightning; had it done so, Franklin would almost certainly have been killed. However, Franklin did notice that loose threads of the kite string were repelling each other and deduced that the Leyden jar was being charged. He moved his hand near the key and observed an electric spark, proving the electric nature of lightning.
De Romas pursued his priority claim to the kite experiment. He had it recognised by the Bordeaux Academy, and then the Académie française in Paris.
Modern controversy
The standard account of Franklin's experiment is disputed by science historian Tom Tucker in 2003. According to Tucker, Franklin never performed the experiment, and the kite as described is incapable of performing its alleged role.
Further doubt about the standard account has been cast by an investigation by the television series MythBusters. In a test using a lightning simulator, the team showed that if lightning had actually hit the kite, Franklin would have received a fatal current through his heart. Nevertheless, they confirmed that certain aspects of the experiment were feasible—specifically, the ability of a kite with sufficiently damp string to receive and send to the ground the electrical energy delivered by a lightning strike.
Because of the danger inherent in the kite experiment, it has become illegal in certain locales.