English: One of the bulbs
Thomas Alva Edison used to discover
thermionic emission (the Edison Effect) in 1884. It consists of an Edison incandescent lamp - an evacuated glass bulb with a hairpin shaped bamboo carbon
filament - with an additional platinum foil electrode
(center, inside the filament) connected to wire terminals. A current is applied to the filament, heating it. Edison found when he connected an ammeter between the filament and the auxiliary electrode, a current would flow, passing through the evacuated space of the bulb from filament to electrode. This current was later found to consist of
electrons. This effect is the basis of
vacuum tube technology, which dominated electronics for 50 years, until the 1970s.