What phenomenon did Thomas Edison observe in a vacuum lamp in 1883?

The phenomenon Thomas Edison observed in a vacuum lamp in 1883 was the Edison effect.

While experimenting with incandescent lamps, Edison placed a metal plate inside a bulb near the heated filament. He found that an electric current could flow between the hot filament and the plate in one direction, even though the plate was not connected through the glass.

Edison patented this arrangement in 1884, but he did not develop its full scientific significance. The effect demonstrated the movement of electrons through a vacuum and later became known as thermionic emission.

The discovery became important in the development of the vacuum tube. In 1904, John Ambrose Fleming used a related principle to create the Fleming valve, an early diode used for detecting radio signals. The Edison effect is therefore a bridge between incandescent-lamp research and electronics.