Electrolytic detector

Lore & Background

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 receivers were not suitable to receive AM signals. The transmitters of the time transmitted information by radiotelegraphy, turning the transmitter on and off with a telegraph key to produce pulses of radio waves for Morse code. Receivers only detected the presence or absence of the radio frequency, producing clicks in the earphone. 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 an amplitude-modulated radio wave.

The simplest way to extract the sound waveform from an AM signal is to rectify it—remove the oscillations on one side of the wave, converting it from alternating current to varying direct current. The variations in the amplitude of the radio wave that represent the sound waveform cause variations in the current, which can be converted to sound by an earphone. To do this a rectifier is required, an electrical component that conducts current in only one direction. It was known at the time that passing current through solutions of electrolytes such as acids could have this unilateral conduction property.

In 1902 Fessenden developed what he called a 'barretter' detector that would rectify an AM signal, but it was not very sensitive. The barretter used a fine platinum wire, called Wollaston wire, manufactured as a platinum core in a silver sheath that had to be stripped off with acid. In the process of stripping some Wollaston wire, Fessenden left it immersed in acid too long, eating away most of the wire until only a tip remained in contact with the solution; he noted that it responded well to radio signals being generated nearby and could be used as a new type of detector. This story was disputed at the time, with credit for the discovery also given to Michael I. Pupin, W. Schloemilch, Hugo Gernsback and others. However, it is apparent that Fessenden was the first to put the device to practical use.

Reader's Guide

The electrolytic detector's significance lies in its role as one of the first rectifying detectors capable of demodulating amplitude-modulated (AM) signals, enabling the reception of sound transmissions. On December 24, 1906, US Naval ships equipped with Fessenden's electrolytic detectors received the first AM radio broadcast from Fessenden's Brant Rock, Massachusetts transmitter, a program of Christmas music. This event marked a milestone in the transition from radiotelegraphy to radiotelephony.

The detector was considered very sensitive and reliable compared to contemporary detectors such as the magnetic detector and the coherer. Its operation relied on the asymmetrical conduction of current through an electrolyte solution (sulfuric or nitric acid) with a fine platinum wire tip. A small DC bias voltage was applied, and the incoming RF current flowed better in one direction, recombining gas bubbles that formed at the tip, while flow in the opposite direction reinforced resistance. This asymmetry allowed rectification of the AM signal.

A practical limitation was that strong atmospheric noise would render the detector insensitive, requiring rebiasing after each strong burst of static interference. A later variant, the sealed-point electrolytic detector (commercially known as the Radioson Detector), had the cell sealed in a glass envelope to prevent spillage or evaporation. The electrolytic detector was superseded by crystal detectors and vacuum tube detectors such as the Fleming valve and Audion (triode) around 1913.

Did You Know?

• The electrolytic detector was first used by Reginald Fessenden in 1903.
• It was one of the first rectifying detectors able to receive AM (sound) transmissions.