Benjamin Franklin. Coloured engraving of Benjamin Franklin (1706-90), American experimental scientist and statesman. Franklin proved the distinction between positive and negative charge. Showing that lightning is comprised of electricity, he invented the lightning conductor to protect buildings. He discovered the Atlantic Gulf Stream and its warm temperature, and suggested that ships use a thermometer to navigate it. He also investigated the ability of different colours to absorb solar heat and invented bifocal spectacle lenses. Franklin was an active politician and was one of the signatories of the American Declaration of Independence in 1776.