During the 1740s, Franklin led many advances in science and entrepreneurship. He invented various types of gadgets, from instruments ranging from the harmonica, to the rocking chair, to the flexible catheter, to the American penny, to conducting the famous kite and key experiment that proved that lightning is electricity. His outstanding research and work in these topics led his 1743 brochure "A Proposal for the Promotion of Useful Knowledge" to become the founding document of the American Philosophical Society which easily made him the richest man in Pennsylvania at age 42 years.