Thomas Edison demonstrated his first incandescent light bulb on this date in 1879. Edison didn't invent the light bulb — incandescent lights had been around for almost 40 years — but he was the first to come up with a practical, long-burning design. He realized he was on the right track by the end of October, when he tested a carbonized filament inside a glass vacuum bulb, which produced a light that burned for more than 13 hours. He kept fiddling with it and modifying it, and each version burned a little bit longer than the one before it; by the time he was ready to reveal it to the public his bulb was burning for 40 hours.
After 14 months of testing, 1,200 experiments, and $40,000, he was finally ready for his first public demonstration. He hung strings of lights inside his lab in Menlo Park, New Jersey, and switched them on and off repeatedly, to the awe and delight of his 3,000 spectators. He said, "We will make electricity so cheap that only the rich will burn candles."