Kite Experiment on Electricity by Benjamin Franklin... and others

Class #10 for 10th graders on December the 11th 2019

Benjamin Franklin's experiment with kite and key. An undated painting of Franklin and his son, William, watching the glowing key after the kite was struck by lightning. Image courtesy of Getty.

 

To dispel [a] myth, Franklin’s kite was not struck by lightning. If it had been, he probably would have been electrocuted. Instead, the kite picked up the ambient electrical charge from the storm.

Here’s how the experiment worked: Franklin constructed a simple kite and attached a wire to the top of it to act as a lightning rod. To the bottom of the kite he attached a hemp string, and to that he attached a silk string. Why both? The hemp, wetted by the rain, would conduct an electrical charge quickly. The silk string, kept dry as it was held by Franklin in the doorway of a shed, wouldn’t.

The last piece of the puzzle was the metal key. Franklin attached it to the hemp string, and with his son’s help, got the kite aloft. Then they waited. Just as he was beginning to despair, Priestley wrote, Franklin noticed loose threads of the hemp string standing erect, “just as if they had been suspended on a common conductor.”

Franklin moved his finger near the key, and as the negative charges in the metal piece were attracted to the positive charges in his hand, he felt a spark.

“Struck with this promising appearance, he immediately presented his knucle [sic] to the key, and (let the reader judge of the exquisite pleasure he must have felt at that moment) the discovery was complete. He perceived a very evident electric spark,” Priestley wrote.

Using the Leyden jar, Franklin “collected electric fire very copiously,” Priestley recounted. That “electric fire”—or electricity—could then be discharged at a later time.

Franklin’s own description of the event appeared in the Pennsylvania Gazette on October 19, 1752. In it he gave instructions for re-creating the experiment, finishing with:

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 completely demonstrated.

Franklin wasn’t the first to demonstrate the electrical nature of lightning. A month earlier it was successfully done by Thomas-François Dalibard in northern France. And a year after Franklin’s kite experiment, Baltic physicist Georg Wilhelm Richmann attempted a similar trial but was killed when he was struck by ball lightning (a rare weather phenomenon).

https://www.fi.edu/benjamin-franklin/kite-key-experiment

In “Opinions and Conjectures,” July 29, 1750, Franklin made the first public suggestion by any investigator that the electrical nature of lightning could be proved experimentally.7 The printing of this paper in Experiments and Observations, April 1751, made the suggestion available to all British electricians, but none of them was sufficiently impressed to attempt the experiment. After Dalibard had translated and published the Franklin pamphlet in Paris in February 1752, he and Delor separately undertook for the first time to carry out the suggestion.8 Dalibard’s efforts met with success on May 10, 1752, as he reported in a long memoir to the French Academy of Sciences three days later. The importance of his achievement, and that of Delor, which followed on May 18, was recognized at once in France, and the news was promptly sent to England.9

Dalibard printed his report in the second edition of his translation of Experiments and Observations, 1756. He stated in the “Avertissement” that he had sent a copy of the report to Franklin; “il en fut charmé et m’envoya avec sa réponse, son premier supplément,” but unfortunately neither this exchange of letters nor any of their other early correspondence has been found. In 1773 Franklin’s French editor Barbeu Dubourg printed a long extract from the Dalibard memoir in his Oeuvres de M. Franklin1 and this was reprinted, without significant change, in the fifth edition of Experiments and Observations the next year.2 Dubourg omitted a long introductory section in which Dalibard summarized the earlier speculations on the causes of lightning and recapitulated the reasoning Franklin had presented in his letter on thundergusts to John Mitchell, April 29, 1749, as the basis for his hypothesis that clouds are electrically charged
https://founders.archives.gov/documents/Franklin/01-04-02-0105