A kite, a key and a thunderstorm

By the 1740s, scientists could create sparks and store electric charge in Leyden jars, but nobody knew whether lightning was the same thing. Franklin suspected it was, and proposed experiments with tall iron rods to draw electricity down from storm clouds.

In June 1752, according to accounts by Franklin and his friend Joseph Priestley, he decided not to wait for a tall church spire to be finished. He made a kite from a silk handkerchief, tied a metal key near the end of the string, and flew it in a thunderstorm in Philadelphia, with his son William helping.

He stood in a shed to keep the silk ribbon in his hand dry. When the loose threads of the string stood on end, he touched his knuckle to the key and felt a spark. He charged a Leyden jar from it. Lightning was electricity.

The kite was probably never struck directly; if it had been, he could have died, as a scientist in Russia did the following year. Franklin used the knowledge at once: he invented the lightning rod and chose not to patent it, so anyone could use it. Buildings around the world are still protected by his idea.