Lee De Forest’s 1907 Audion vacuum tube is widely credited with kickstarting the modern electronics industry in general, but it would take the much smaller, lower-power and cooler transistor to shift it into high gear.
The thermonic triode, a vacuum tube similar to the one pictured, works by using heat to modify the conductive properties of a fillament. In 1880, Thomas Edison discovered that, when a parallel current was applied to the lament circuit, the charge in the hot lament could be amplified, or cut off. Edison used his “thermonic diode” to power the first telegraph stations. The triode added a charged grid that could modify the current.