English: Caption: "The RJ6 Receiving Set Mfd. by The De Forest Radio Tel. & Tel. Co." A one-tube radio receiver that used the Audion (triode) vacuum tube invented by Lee De Forest in 1906. The amplifying ability of the Audion was not recognised until 1913, so this is one of the earliest amplifying radio receivers. The Audion is visible on the front, mounted upside down, as Audions always were, with the delicate filament loop in the bulb hanging down so it would not touch the grid. The Audion served as both amplifier and detector, to rectify the radio signal and extract the audio from the carrier wave. The multiposition switch in upper center adjusted the plate voltage; Audions used about 20-30 V and were very sensitive to the voltage applied. The lower center switch controlled the filament current. The larger dial at right was the tuning. To the left are the headphones used to listen; the set did not produce enough power to drive a loudspeaker.