Electricity

Benjamin Franklin is often noted for his "discovery" of electricity. Of course, electricity has always existed, but Franklin was the first to "harness" electricity and make it a usable source of energy. In the 1750s he began experimenting with electricity, trying to prove certain theories he had about its nature and its relationship to lightning. His interest in lightning possibly grew out of his meterological observations in the 1740s, when he began charting storms. "Playing" with static electricity was in vogue among Enlightenment intellectuals, who used "toys" like electricity tubes -- a glass rod and piece of silk -- and static electricity generators. Franklin connected the dots between electricity and lightning, proving lightning is electric current, with his famous kite experiment in 1752, where he attached a key to a kite during a thunderstorm and observed the key discharged electricity when the kite was struck by lightning. He also invented the lightning rod and first used the words charge, discharge, condenser, conductor, battery and electric shock to describe electrical phenomena.

Inventions

The lightning rod was not Franklin's only invention, although he considered it to be his most important, because it drastically reduced the danger of house fires caused by lightning. (Incidentally, Franklin's idea for the first fire department came out of his concern for fire prevention.) He also invented swimming fins; the Franklin stove, which reduced smoke and fuel consumption while increasing heat output; bifocal lenses; the glass armonica, a sort of xylophone made of glass hemispheres played by rubbing a finger around the rims of the glasses and for which Beethoven and Mozart composed music; and flexible catheters. The public library was not his only library-related invention, either. Franklin invented a library chair that had a pedal-operated fan and a seat that doubled as a step stool. He also invented an extension arm, a long pole with a clamp users could open and close using a piece of string, that he used to retrieve items on high shelves.

Other Scientific Contributions

The Gulf Stream
Franklin grew interested in the Atlantic Ocean currents on his frequent voyages between the colonies and England. He crossed the Atlantic eight times -- many more than most people in the 18th century -- and measured the temperature of the water. He was one of the first people to attempt to chart the Gulf Stream.

Medicine
The first public hospital, the Pennsylvania Hospital, was an idea of Franklin's friend Thomas Bond. Franklin helped him raise money and support for the hospital, and hit upon the idea of matching grants -- combining government and private funds. Franklin also identified lead as the source of several ailments, some of them common to printers. He observed that certain medical conditions occured more often among men whose occupations exposed them to significant amounts of lead, such as printers, plumbers and glazers. His observations of sailors, who spent all their time wearing wet clothes but did not often catch colds, led him to debunk the common myth that wet clothes caused colds. He correctly determined that close, poorly ventilated quarters were more likely the cause and guessed that colds were transmitted through the air, as people "breathe in each other's transpiration."