Example of single tube triode grid leak receiver from 1920, the first type of amplifying radio receiver. In the left picture the grid leak resistor and capacitor are labeled.
A grid leak detector is an electronic circuit that demodulates an amplitude modulated alternating current and amplifies the recovered modulating voltage. The circuit utilizes the non-linear cathode to control grid conduction characteristic and the amplification factor of a vacuum tube.[1][2] Invented by Lee De Forest around 1912, it was used as the detector (demodulator) in the first vacuum tube radio receivers until the 1930s.