The kite experiment is a scientific experiment in which a kite with a pointed, conductive wire attached to its apex is flown near thunder clouds to collect electricity from the air and conduct it down the wet kite string to the ground. It 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, and with further experiments on the ground, to demonstrate that lightning and electricity were the result of the same phenomenon.
Speculations of Jean-Antoine Nollet had led to the issue of the electrical nature of lightning being posed as a prize question at Bordeaux in 1749. In 1750, it was the subject of public discussion in France, with a dissertation of Denis Barberet receiving a prize in Bordeaux; Barberet proposed a cause in line with the triboelectric effect. The same year, Franklin reversed his previous skepticism of electrical lightning's attraction to high points. The physicist Jacques de Romas also wrote a mémoire with similar ideas that year, and later defended them as independent of Franklin's.
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. However, the spire at Christ Church was not added until 1754.
Franklin's kite experiment was performed in Philadelphia in June 1752, according to the account by Priestley. 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.
According to the 1767 Priestley account, 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 while his son assisted him to fly the kite from the shelter of a nearby shed. This enabled Franklin and his son to keep the silk string of the kite dry to insulate them while the hemp string to the kite 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. "At this key he charged phials, and from the electric fire thus obtained, he kindled spirits, and performed all other electrical experiments which are usually exhibited by an excited globe or tube." The kite was not struck by visible lightning; had it been, 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.
The kite experiment was described in The Pennsylvania Gazette on October 19, 1752 as follows:
Philadelphia, October 19
As frequent Mention is made in the News Papers from Europe, of the Success of the Philadelphia Experiment for drawing the Electric Fire from Clouds by Means of pointed Rods of Iron erected on high Buildings, &c. it may be agreeable to the Curious to be inform'd, that the same Experiment has succeeded in Philadelphia, tho' made in a different and more easy Manner, which any one may try, as follows.
Make a small Cross of two light Strips of Cedar, the Arms so long as to reach to the four Corners of a large thin Silk Handkerchief when extended; tie the Corners of the Handkerchief to the Extremities of the Cross, so you have the Body of a Kite; which being properly accommodated with a Tail, Loop and String, will rise in the Air, like those made of Paper; but this being of Silk is fitter to bear the Wet and Wind of a Thunder Gust without tearing. To the Top of the upright Stick of the Cross is to be fixed a very sharp pointed Wire, rising a Foot or more above the Wood. To the End of the Twine, next the Hand, is to be tied a silk Ribbon, and where the Twine and the silk join, a Key may be fastened. This Kite is to be raised when a Thunder Gust appears to be coming on, and the Person who holds the String must stand within a Door, or Window, or under some Cover, so that the Silk Ribbon may not be wet; and Care must be taken that the Twine does not touch the Frame of the Door or Window. As soon as any of the Thunder Clouds come over the Kite, the pointed Wire will draw the Electric Fire from them, and the Kite, with all the Twine, will be electrified, and the loose Filaments of the Twine will stand out every Way, and be attracted by an approaching Finger. And when the Rain has wet the Kite and Twine, so that it can conduct the Electric Fire freely, you will find it stream out plentifully from the Key on the Approach of your Knuckle. At this Key the Phial may be charg'd; and from Electric Fire thus obtain'd, Spirits may be kindled, and all the other Electric Experiments be perform'd, which are usually done by the Help of a rubbed Glass Globe or Tube; and thereby the Sameness of the Electric Matter with that of Lightning compleatly demonstrated.
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.
As for Franklin, who doubted in 1749 that the lightning was attracted by the spikes, he wrote ... the following year: 'When the electrified clouds pass over a country, the summits of mountains and trees, the high towers, the pyramids, the masts of the ships, the chimneys, etc., like so many eminences and peaks, attract the electric fire, and the whole cloud is unloaded.'