What phenomenon did Thomas Edison discover while testing incandescent lamps in 1883?

While testing incandescent lamps in 1883, Thomas Edison discovered the phenomenon later called the Edison effect.

Edison observed that an electric current could pass through a vacuum from a heated filament to a separate metal plate, even though the plate was not connected by a solid conductor. He patented the observation, but he did not develop its full theoretical significance.

The effect was later explained as thermionic emission: heated materials release electrons. In the early 20th century, John Ambrose Fleming used the phenomenon in the vacuum-tube diode, and it became fundamental to radio, electronics, and early computers.

The Edison effect is not the photoelectric effect, in which light causes electrons to be emitted. The two phenomena both involve electrons but arise from different physical causes.