What did Thomas Edison observe that became known as the Edison effect?

In 1883, Thomas A. Edison observed a phenomenon while experimenting with his incandescent lightbulbs that became known as the Edison effect. He noticed a bluish glow inside some of his early bulbs and discovered that an electric current would flow from the hot filament (cathode) to a separate metal plate (anode) placed inside the bulb, provided the anode was positively charged relative to the filament.12

Later scientific study revealed that this effect is caused by thermionic emission, where heat provides electrons with enough energy to escape the surface of the hot filament. These electrons then travel through the vacuum to the positively charged anode, creating a current that does not exist if the anode is negatively charged. This discovery was a foundational development in the history of electronics, providing the essential principle for the creation of electron tubes.2