By the end of the 1730s, Franklin was well known as a civic leader. His writings, especially those of Poor Richard, were famous. Yet relatively few people knew about Franklin's scientific interests. Within twenty years he would be called the greatest scientist in America, with a reputation that rivaled Newton's. His first major step toward scientific fame came in 1743, when he wrote an essay entitled "A Proposal for Promoting Useful Knowledge," which led to the establishment of the American Philosophical Society. It was America's first scientific society.
In 1745, the Library Company received a pamphlet describing German experiments in electricity from a London merchant named Peter Collinson. At this time little was known about the nature of electricity, and Franklin was eager to discover more. With the help of several colleagues, he soon proved that there was only one kind of electricity–most people at the time thought there were two–and that when objects were electrified, nothing new was created or lost. Franklin demonstrated that the electricity is simply rearranged. This became known as Franklin's law of the conservation of charge and has since become a fundamental law of electricity.
Franklin went on to suggest that lightning is electrical. In March 1750, he proposed the lightning rod as a way to test this theory and to protect tall buildings from being damaged by lightning. His letter describing the results were published in London as Experiments and Observations on Electricity, and soon translated into French. Having been unable to build a lightning rod, Franklin found a new way to test his theory about lightning. In June 1752, during a lightning storm, he flew a kite connected by a hemp string to a Leyden jar, a device that registered electrical charge. When he saw the fibers of the hemp string stand up, he knew the kite had been electrified, and his theory confirmed. This experiment, along with experiments conducted in France based on his research, confirmed Franklin's fame as a scientist throughout Europe.
By 1748 Franklin was wealthy enough to retire. He let a business partner named David Hall take over the printing business in return for a share of the profits and devoted himself fulltime to science and civic leadership. He continued his good works, leading a campaign in 1749 to found an academy; it would later become the University of Pennsylvania. He also proposed and helped to sponsor the Pennsylvania Hospital, which opened on February 6, 1752. It was America's first.
Though busy with civic projects and scientific experiments, Franklin also ventured into politics. In 1747, when Pennsylvania was threatened by pirates on the Delaware River and by French and Indian attacks on the western frontier, Franklin led a campaign to establish a militia. The militia met on January 1, 1748, and elected Franklin its commander. He declined, claiming he lacked military experience, and he served instead as a common soldier. By the end of the year, that threat was gone and the militia disbanded, but not before Franklin had become a hero. He was so popular, in fact, that some of the more powerful people in the colony began to fear him.
When Franklin retired, he was comfortably wealthy. He was hardly rich, however, and seemed to be turning his back on a very lucrative business. While he remained very ambitious, it clearly was not money he wanted. He wanted, above all, to be respected as an intellectual. His first love was "philosophical investigation"–today we call it science.