What term names the one-way current discovered in an Edison vacuum lamp?

The one-way current discovered in an Edison vacuum lamp is called the Edison effect.

Edison observed in 1883 that current could flow between a heated filament and a separate metal plate inside a bulb, but only in one direction. He did not develop a practical use for the observation at the time, and the effect was not central to his lighting business.

The discovery later became important in electronics. Scientists identified the process as thermionic emission: heat causes electrons to leave a metal surface. John Ambrose Fleming used the principle in 1904 to create the thermionic diode, an early vacuum-tube device.

The Edison effect is therefore an example of an observation whose major technological importance emerged years after the original experiment. It helped lead to electronic rectifiers, amplifiers, and early radio technology.