June/July 2002 | Volume 53, Issue 3
In June of 1752, in Philadelphia, Benjamin Franklin performed what may be the most famous scientific experiment of all time by flying a kite in a thunderstorm. In so doing, he verified his theory that lightning is a form of electricity. The experimental apparatus consisted of an ordinary kite with a footlong metal spike attached to its end to attract lightning. At the bottom of the kite string was a key, which absorbed the electricity from the lightning. To keep himself from being electrocuted, Franklin put a length of silk thread between the key and his hand. He stood inside a shed and flew the kite through a window to keep the silk dry and thus non-conducting.
When lightning struck, the spike, the kite, the string (which was wet, allowing it to conduct electricity easily), and the key all became electrifled. A finger placed near the string attracted its fibers; a knuckle next to the key brought forth sparks. The electricity that was captured in the key could be used to perform all the same experiments and demonstrations as ordinary electricity.
Franklin had suggested the equivalence of lightning and electricity as early as 1749, but he was not the first to do so. Nor was he the first to prove it experimentally. In France, Thomas-François Dalibard and a fellow scientist named Delor each had verified the conjecture in May of 1752. In their method, which Franklin had suggested, a metal spike was placed at the top of a tall structure, and the experimenter drew sparks from a thundercloud by bringing together a rod and a wire. It is very unlikely that Franklin knew about the French experiments when he found his more picturesque way of getting a spike into the sky.
One practical benefit that came out of the experiments was the protective lightning rod. Franklin gave instructions for building such a device in his 1753 edition of Poor Richard’s Almanack . Useful applications of electricity itself, however, would have to wait nearly a century until the introduction of the telegraph during the 1840s.