A triode is an electronic amplifying vacuum tube (valve) consisting of three electrodes inside an evacuated glass envelope: a heated filament (cathode), a grid, and a plate (anode).
Invented in 1906 by Lee De Forest by adding a grid to the Fleming valve, the triode was the first electronic amplification device. Its invention founded the electronics age, making possible amplified radio technology and long-distance telephony. Triodes were widely used in consumer electronics devices such as radios and televisions until the 1970s when they were replaced by transistors.
This is a triode, type R (R representing receiving). It has a pure tungsten filament operating at 4-volts, 0.65 amps. The R valve was a direct derivative of the French TM valve and was produced in the UK by several companies. This example was of about 1920 manufacture, probably by the Marconi-Osram Valve Company. It was widely used in radio receivers up to the mid-1920s.