Electrolytic detector

The electrolytic detector, or the bare-point electrolytic detector as it was also called, was a type of detector (demodulator) used in early radio receivers. First used by Canadian radio researcher Reginald Fessenden in 1903, it was used until about 1913, after which it was superseded by vacuum tube detectors such as the Fleming valve and the Audion. It was considered very sensitive and reliable compared to other detectors available at the time such as the magnetic detector and the coherer. It was one of the first rectifying detectors, able to receive AM (sound) transmissions. On December 24, 1906, US Naval ships with radio receivers equipped with Fessendon's electrolytic detectors received the first AM radio broadcast from Fessenden's Brant Rock, Massachusetts transmitter, consisting of a program of Christmas music.

Fessenden, more than any other person, is responsible for developing amplitude modulation (AM) radio transmission around 1900. While working to develop AM transmitters, he realized that the radio wave detectors used in existing radio receivers were not suitable to receive AM signals. The radio transmitters of the time transmitted information by radiotelegraphy; the transmitter was turned on and off by the operator using a switch called a telegraph key producing pulses of radio waves, to transmit text data using Morse code. Thus receivers didn't have to extract an audio signal from the radio signal, but only detected the presence or absence of the radio frequency to produce "clicks" in the earphone representing the pulses of Morse code. The device that did this was called a "detector". The detector used in receivers of that day, called a coherer, simply acted as a switch, that conducted current in the presence of radio waves, and thus did not have the capability to demodulate, or extract the audio signal from, an amplitude modulated radio wave.

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