Benjamin Franklin's Kite Experiment (1752): Taming Lightning

Benjamin Franklin’s kite experiment, carried out in Philadelphia in June 1752, showed that thunderclouds carry electric charge and that lightning is a giant electric spark. Flying a silk kite in a storm, Franklin drew sparks from a key on the string and stored the charge in a Leyden jar. Together with experiments in France a few weeks earlier, it turned lightning from a mysterious force into a natural phenomenon that could be explained and guarded against with the lightning rod.

Background: Franklin the electrician

Benjamin Franklin, born in Boston in 1706, was a successful Philadelphia printer, publisher and civic organiser when he became fascinated by electricity in the mid-1740s. He had seen demonstrations by the travelling lecturer Archibald Spencer, and in 1747 he received a glass tube for experiments from Peter Collinson, a London merchant and Fellow of the Royal Society who supplied the Library Company of Philadelphia.

Working with friends including Ebenezer Kinnersley, Thomas Hopkinson and Philip Syng, Franklin experimented intensively. He proposed that electricity was a single “fluid” that could be in excess (positive, or plus) or deficit (negative, or minus) in a body, and that the total amount was conserved. Words he introduced or popularised in English, such as “battery”, “charge” and “conductor”, are still used today.

Franklin also studied the Leyden jar, invented in 1745–1746, and showed that its charge was bound to the glass between its coatings. He sent his findings to Collinson in a series of letters.

The idea that lightning is electricity

Many people had noticed that sparks and lightning look and sound alike. Franklin went further. In his notes of November 1749 he listed the shared features of lightning and electricity: light, crooked paths, noise, the ability to kill animals and melt metals, and a tendency to strike pointed or projecting objects.

He had also found that sharp metal points could quietly draw charge away from an electrified body. In a letter of 1750 he proposed a test: a sentry box on a high tower, with an iron rod rising about 20 to 30 feet. A person standing on an insulating stand inside could draw sparks from the rod if a charged cloud passed over.

Collinson arranged for Franklin’s letters to be published in London in 1751 as Experiments and Observations on Electricity, Made at Philadelphia in America. The pamphlet attracted modest notice in England but caused a sensation in France after it was translated in 1752.

The French experiments of May 1752

The naturalist Georges-Louis Leclerc, comte de Buffon, encouraged the translation, made by Thomas-François Dalibard. Dalibard then set up Franklin’s proposed experiment at Marly-la-Ville, north of Paris, with an iron rod about 40 feet (12 metres) long, insulated at its base.

On 10 May 1752, while Dalibard was away, a thundercloud passed over. A retired dragoon named Coiffier, whom he had left in charge, drew sparks from the rod, and the local priest, Raulet, came to watch. Dalibard reported the result to the Academy of Sciences three days later.

A few days after that, the physicist Delor repeated the experiment in Paris, and it was soon performed before Louis XV. Franklin’s hypothesis had been confirmed in Europe before he tried his own test, and he became famous across the continent.

The kite in Philadelphia

Franklin had been waiting for the steeple of Christ Church in Philadelphia to be completed so that he could use it for the sentry-box test. Instead, he realised that a kite could carry a conductor high into the sky.

According to Franklin’s own account, he made a kite from two light cedar strips and a large silk handkerchief, since silk would survive rain and wind. A sharp wire projected about a foot above the top. The kite was flown on ordinary twine, which conducted electricity when wet; at the lower end Franklin tied a silk ribbon to hold in his hand, and at the junction of twine and ribbon he hung a key.

Franklin was to stand under a door or window, or in a shed, so that the silk ribbon stayed dry and insulated him from the charge. When a thundercloud came over, the loose fibres of the twine stood out and repelled each other. Bringing a knuckle near the key produced a spark. He also charged a Leyden jar, called a phial, from the key, and the stored “electric fire” behaved exactly like charge from a friction machine.

The date of the experiment is not recorded. Franklin did not publish an account until a letter dated 19 October 1752, printed in the Pennsylvania Gazette that day and read at the Royal Society later in the year. Joseph Priestley’s History and Present State of Electricity (1767), based on information from Franklin, placed the flight in June 1752 and named William Franklin, his son, as his helper.

Doubts and dangers

Because Franklin waited months to report the kite, and wrote his account in the form of instructions rather than a dated narrative, a few writers have doubted that he ever flew it. Most historians of science, including I. Bernard Cohen, accept that he did, pointing to the practical details and to Priestley’s independent account.

There is a separate question of priority. The French experiments took place in May, while Franklin’s kite flew in June, probably before news of Marly reached America. Franklin had designed the French test, so both successes are generally credited to his idea.

The kite was never struck by lightning; a direct strike would very probably have killed Franklin. The experiment collected charge from the electrified atmosphere of the storm, which was dangerous enough.

On 6 August 1753 (26 July in the Russian calendar), the Baltic German physicist Georg Wilhelm Richmann was killed in St Petersburg while measuring atmospheric electricity during a storm with an insulated rod. A ball of lightning reportedly struck him on the forehead. He is often described as the first person known to have died while experimenting with electricity.

The lightning rod

Franklin’s practical aim was protection. Lightning regularly destroyed church steeples, barns and ships, and gunpowder stores could explode. He argued that a pointed iron rod fixed to the top of a building, and connected by a wire or rod to the ground, would either quietly draw charge from a cloud or carry a strike harmlessly to earth.

He published simple instructions in Poor Richard’s Almanack for 1753 and installed a rod on his own Philadelphia house, connected to bells that rang when a charged cloud passed. Rods spread rapidly in the American colonies and more slowly in Europe, where some clergy and others objected that they interfered with divine providence.

Debate also arose over whether rods should be pointed, as Franklin advised, or end in a ball. In 1777, after a gunpowder magazine at Purfleet was struck, the pointed rods that a Royal Society committee including Franklin had recommended in 1772 were challenged by Benjamin Wilson, who favoured blunt terminals. King George III, by then at war with the American colonies, sided with Wilson, according to a widely told story whose details are uncertain.

Fame and recognition

The experiments made Franklin one of the best-known scientists of his age. The Royal Society awarded him its Copley Medal in 1753 and elected him a Fellow in 1756, and universities including Harvard, Yale, St Andrews and Oxford gave him honorary degrees.

That reputation helped him greatly as a diplomat. When he arrived in France in 1776 to seek support for American independence, after helping draft the Declaration of Independence, he was already a celebrity. The French statesman Turgot summed up his fame in a Latin epigram: “He snatched lightning from the sky and the sceptre from tyrants.”

Myths and facts

Myth: lightning struck the kite and electrified the key. No direct strike occurred. Franklin collected charge from the storm cloud, which produced only small sparks.

Myth: Franklin discovered electricity. Electricity had been studied for centuries, and Franklin built on the work of many others. His contribution was to explain it better and to show that lightning is electrical.

Myth: a small boy held the kite. William Franklin was about 21 in 1752. Popular prints and paintings, including a Currier and Ives lithograph of 1876, show him as a child for dramatic effect.

Myth: Franklin performed the first lightning experiment. The first successful test was at Marly-la-Ville in France on 10 May 1752, following Franklin’s published proposal.

Timeline

• 1747: Franklin receives a glass tube from Peter Collinson and begins serious electrical experiments.
• 1750, July: In a letter, Franklin proposes the sentry-box test for lightning.
• 1751: Experiments and Observations on Electricity is published in London.
• 1752, 10 May: Franklin’s proposed experiment succeeds at Marly-la-Ville, France.
• 1752, June: Franklin flies his kite in Philadelphia with his son William.
• 1752, 19 October: Franklin’s account of the kite appears in the Pennsylvania Gazette.
• 1753: Franklin receives the Copley Medal; Poor Richard’s Almanack describes the lightning rod.
• 1753, 6 August: Georg Wilhelm Richmann is killed during a storm experiment in St Petersburg.

Why it matters

The kite experiment and its French counterpart brought one of nature’s most feared forces into the realm of science. They proved that the same laws applied to sparks in a laboratory and to lightning in the sky, gave the world a practical protection in the lightning rod, and helped lay the groundwork for the electrical science that would follow with Volta’s battery.