Franklin, after twenty years of assiduous business and fortunate investments, was able to retire on an income of about four thousand dollars a year, which in those times was a comfortable independence anywhere. He retired with the universal respect of the community both as a business man and a man of culture. Thus far his career was not extraordinary, not differing much from that of thousands of others in the mercantile history of this country, or any other country. By industry, sagacity, and thrift he had simply surmounted the necessity of work, and had so improved his leisure hours by reading and study as to be on an intellectual equality with anybody in the most populous and wealthy city in the country. Had he died before 1747 his name probably would not have descended to our times. He would have had only a local reputation as a philanthropical, intelligent, and successful business man, a printer by trade, who could both write and talk well, but was not able to make a better speech on a public occasion than many others who had no pretension to fame.
But a new career was opened to Franklin with the attainment of leisure and independence,--the career of a scientific investigator. The subject which most interested him was electricity, just then exciting great interest in Europe. In 1746 he attended in Boston a lecture on electricity by Dr. Spence, of Scotland, which induced him to make experiments himself, the result of which was to demonstrate to his mind the identity of the electrical current with lightning. What the new, mysterious power was, of course he could not tell, nor could any one else. All he knew was that sparks, under certain conditions, were emitted from clothing, furs, amber, jet, glass, sealing-wax, and other substances when excited by friction, and that the power thus producing the electric sparks would repel and attract. That amber, when rubbed, possesses the property of attracting and repelling light bodies was known to Thales and Pliny, and subsequent philosophers discovered that other substances also were capable of electrical excitation. In process of time Otto Guericke added to these simple discoveries that of electric light, still further established by Isaac Newton, with his glass globe. A Dutch philosopher at Leyden, having observed that excited electrics soon lost their electricity in the open air, especially when the air was full of moisture, conceived the idea that the electricity of bodies might be retained by surrounding them with bodies which did not conduct it; and in 1745 the Leyden jar was invented, which led to the knowledge that the force of electricity could be extended through an indefinite circuit. The French savants conveyed the electric current through a circuit of twelve thousand feet.