Thomas Edison unveiled the first incandescent light bulb, which burned for 40 hours, on this day in 1879. Although the idea for converting electricity into light was first investigated in 1848 by English physicist Sir Joseph Swan, Edison made it commercially viable. In a light bulb, electricity passes through a thin wire, which heats up and glows. Swan had trouble preventing the wire from oxidizing and quickly burning out.
When Edison turned to the problem in 1878, it was possible to make a bulb that had a good vacuum, thus protecting the filament from oxidation. Edison discovered that a charred length of cotton thread worked better than a wire. After demonstrating his light bulb, Edison invented the generating, switching, and transmitting devices needed to supply electricity to many bulbs at once. By 1882 his first power station was operating in New York City and lighting 400 incandescent bulbs for 85 customers.
[Source: Emily McMurray, Ed. Notable Twentieth Century Scientists. Gale Research Inc. ITP. 1995.]