In 1747 Franklin made what is generally considered his chief contribution to science. One of his correspondents, Collinson (a Fellow of the Royal Society and a botanist interested in useful plants, through whom the vine was introduced into Virginia), had sent to the Library Company at Philadelphia one of the recently invented Leyden jars with instructions for its use. Franklin, who had already seen similar apparatus at Boston, and his friends, set to work experimenting. For months he had leisure for nothing else. In this sort of activity he had a spontaneous and irrepressible delight. By March, 1747, they felt that they had made discoveries, and in July, and subsequently, Franklin reported results to Collinson. He had observed that a pointed rod brought near the jar was much more efficacious than a blunt rod in drawing off the charge; also that if a pointed rod were attached to the jar, the charge would be thrown off, and accumulation of charge prevented. Franklin, moreover, found that the nature of the charges on the inside and on the outside of the glass was different. He spoke of one as plus and the other as minus. Again, "We say _B_ (and bodies like-circumstanced) is electricized positively; _A_ negatively." Dufay had recognized two sorts of electricity, obtained by rubbing a glass rod and a stick of resin, and had spoken of them as vitreous and resinous. For Franklin electricity was a single subtle fluid, and electrical manifestations were owing to the degree of its presence, to interruption or restoration of equilibrium.
His mind, however, was bent on the use, the applications, the inventions, to follow. He contrived an "electric jack driven by two Leyden jars and capable of carrying a large fowl with a motion fit for roasting before a fire." He also succeeded in driving an "automatic" wheel by electricity, but he regretted not being able to turn his discoveries to greater account.
He thought later--in 1748--that there were many points of similarity between lightning and the spark from a Leyden jar, and suggested an experiment to test the identity of their natures. The suggestion was acted upon at Marly in France. An iron rod about forty feet long and sharp at the end was placed upright in the hope of drawing electricity from the storm-clouds. A man was instructed to watch for storm-clouds, and to touch a brass wire, attached to a glass bottle, to the rod. The conditions seemed favorable May 10, 1752; sparks between the wire and rod and a "sulphurous" odor were perceived (the manifestations of wrath!). Franklin's well-known kite experiment followed. In 1753 he received from the Royal Society a medal for the identification and control of the forces of lightning; subsequently he was elected Fellow, became a member of the Académie des Sciences, and of other learned bodies. By 1782 there were as many as four hundred lightning rods in use in Philadelphia alone, though some conservative people regarded their employment as impious. Franklin's good-will, clearness of conception, and common sense triumphed everywhere.