Single tube triode radio receiver from 1924, with parts labeled. Invented by Lee De Forest with the triode around 1912, this was the earliest vacuum tube receiver. The tube serves as a grid-leak detector to rectify the radio signal to extract the audio modulation (sound) from it, and also amplify it. The grid leak, a megohm resistor and capacitor in parallel connected in series with the grid, served to bias the grid with a negative DC voltage near cutoff, so it only conducted current on the positive half cycles of the RF cycle. It used a UV-200 tube.