English: Plate current vs voltage curve of an early tetrode vacuum tube, an RCA 24A from 1930 or earlier, showing region of negative slope (negative differential resistance) used in dynatron oscillators. When electrons from the cathode strike the plate, they can knock other electrons out of it which are attracted to the high potential on the screen grid. Due to this secondary emission of electrons, the plate current can actually decrease with an increase in plate voltage. This negative resistance is used in the dynatron oscillator circuit to create oscillations in a tuned circuit. Modern tetrodes have their plate treated to inhibit secondary emission, so they don't have this negative resistance region.