English: The liquid "barretter", an electrolytic diode invented in 1902 by Canadian electrical engineer Reginald Fessenden, used as a radio wave detector in early radio receivers during the wireless telegraphy era. It was one of the first radio detectors which could rectify (demodulate) AM (sound) radio transmissions. Fessenden used it in his historic first AM radiotelephone transmissions in 1906. It should not be confused with the hot wire barretter, a different device also invented by Fessenden.

It consists of a cup (right) containing dilute nitric acid, with a fine platinum wire touching the surface. When the wire was positive with respect to the cup it would conduct current, but when it was negative minute hydrogen bubbles would form on the electrode so it would be nonconductive. Therefore it would only conduct current in one direction, so it would rectify a radio signal applied to it. The support post (left) has a micrometer thumbscrew to adjust the height of the wire so it just touches the electrolyte solution. The wire slowly dissolved in the acid and periodically more wire had to be let down from the reel (center top).