INVENTING the DIODE and the TRIODE

by

Dick Reiman, Historian

The emission of electrons from a heated surface such as a diode or triode cathode was probably first observed by Thomas Edison in 1883. In his experiments with carbon filament lamps, he noted that carbon was deposited on the inner surface of the bulb when the filament was operated at a high voltage. He suspected that some unknown electrical force was the cause and he confirmed this by connecting a small metal plate suspended inside the bulb to an external battery. When the plate was connected to the positive battery pole, he observed a flow of current with a sensitive current-measuring instrument. He had created the first diode. The electron had not yet been discovered (by J.J. Thompson in the 1890's at Cambridge University) and Edison filed a patent application, but pursued it no further.

The discovery of a practical application of the diode was made by an English engineer, Sir John

Ambrose Fleming, in 1904, as a consultant to the Marconi Wireless Telegraph Company, to improve transmitting and receiving apparatus. If the alternating current produced by electromagnetic waves of the wireless could be converted to direct current by rectifying it, a more sensitive detector of radio signals would result. Felming had read of Edison's work and thought the diode might be the answer. Fleming built a number of diodes but these were only modest improvements over the crystal detectors that had been used previously. Nevertheless, Fleming received recognition for his device (which was called a "Fleming Valve"), but what was needed was an amplifier. The amplifier was the triode, a diode with a grid structure added. By applying a low-power input signal to the grid, an output signal of much greater power could be produced at the anode. The electron tube could now be used in communication and broadcasting systems.

Lee DeForest of Council Bluffs, Iowa, son of a Congregational minister and a graduate of the Sheffield Technical School of Yale University, became the inventor of the triode in 1906. DeForest was also trying to improve radio receivers by amplifying. Surprisingly, DeForest did not have a clear idea of how the triode worked since he beleived that amplification was due to or aided by the gas in the tube. This faulty explanation caused him trouble in patent litigations which followed his invention.

Spark transmitters used by Hertz and Marconi were of low efficiency and had a wide band of frequency components which made a terrible racket (the radio operator on board ships was called "Sparks"). English physicist Sir Oliver Lodge discovered in 1897 that circuits which were tuned to resonate at the transmitter's frequency could smooth the energy bursts into a continuous wave. The receiver thereby could also be tuned to a single frequency. This circuitry for receivers was the invention of the superheterodyne circuit for radio receivers by Maj. Edwin Howard Armstrong. AM radio broadcasting would follow.