When British electrical engineer John Ambrose Fleming (1849-1945) invented the first thermionic diode, he had no idea of the impact it would have on the technology of the twentieth century.
The diode is used in many electrical and electronic systems. In its early valve form, the diode used thermionic emission—the flow of electrons within a sealed vacuum—to create a one-way valve for electrical current. This is critical in regulating voltages, processing high-frequency signals, and in converting AC (alternating current) to DC (direct current).
Predictably, Thomas Edison, was one of the first to discover the underlying principles. In 1883 he created a kind of diode by modifying an electric light bulb. He patented what he called his "Edison Effect," but ultimately saw little potential in its development. In 1904, having acquired a number of Edison Effect bulbs from the United States, Fleming developed an "oscillation valve," which he used to convert radio waves into a one-way flow of current that could be measured on a galvanometer. His invention, known as the "Fleming Valve"—the term diode only came to be used much later—would make the early radio and phonographic electronics industries a commercial reality.
For much of the twentieth century, thermionic valve diodes were used in analog signal applications and as rectifiers in power supplies. The development of the smaller, more predictable, transistor in 1947 rendered the valve obsolete for most of its existing uses. Today, valve diodes are only used in niche applications, such as guitar amplifiers.
