An early triode, the Moorhead electron relay, was manufactured by Otis Moorhead beginning in 1916 as a copy of the Cunningham "Audiotron", under Lee De Forest's Audion patents. This version is from around 1919. It is a "soft" vacuum triode like the Audion, with some residual air left in the tube. The ionization of the gas when a voltage was applied gave it greater sensitivity as an AM detector, but limited the plate voltage so it couldn't produce much power. It consisted of a partially evacuated glass tube with a tungsten filament down the center of the tube, surrounded by a helical wire grid, surrounded by a cylindrical aluminum plate.