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Franklin wasn't the first to propose a kinship between harmless sparks and a lightning bolt. But he was the first to suggest an experiment to prove it. The Royal Society of London published his proposal, yet it was the French who actually put it to the test. The experiment Franklin proposed, which he first revealed in a letter to his English agent in July 1750, called for installing on a high place, like a steeple, a sentry box with a metal pole extending from its roof. If an electrified storm cloud passed overhead, Franklin said, the pole--preferably sharpened at the end--would pull out a small amount of the cloud's "fire." Or to put it in modern terms, it would induce an electrical charge in the pole. An observer in the sentry box could detect the charge by touching the pole with an insulated ground wire and drawing sparks. Or if the pole itself was grounded, it would extract all the cloud's "fire" in a lightning bolt and sweep it harmlessly into the earth. Franklin had created the lightning rod.
But before Franklin learned, late in August 1752, of the French success with his experiment that spring (and of the King's compliments to Monsieur Franklin), he set about undertaking it himself in June of that year--with a special wrinkle. The steeple he had hoped to use was unfinished, and he decided he could prove his case just as easily with a wired kite. It would rise even higher in the sky. So why did he do it on the sly? Joseph Priestley, the British chemist and a Franklin crony, later explained, "... dreading the ridicule which too commonly attends unsuccessful attempts in science, he communicated his intended experiment to nobody but his son, who assisted him in raising the kite."
When the secret finally got out, it had sweeping repercussions. As Harvard historian of science I. Bernard Cohen (who died June 20) has pointed out, Franklin's experiment showed that electricity was not just an amusing "bizarrery" but a force of nature, like gravity. It also illustrated an Enlightenment ideal: that pure science--science done for the joy of exploring nature--could have enormous practical consequences, as shown by the lightning rod. The invention drastically reduced a perennial fire threat to churches and other tall structures. Most profoundly, it shook the belief that lightning was a sign of God's displeasure.
What Franklin modestly described as his "electrical amusements" made him the world's most famous scientist. The German philosopher Immanuel Kant called him the "new Prometheus." Most important, Franklin's fame helped open French hearts--and purse strings--when years later he came calling at Louis XVI's court on behalf of his embattled young nation. As the French financier Turgot would say of the kite flyer from Philadelphia, "He snatched lightning from the sky and the scepter from tyrants."