However the dynatron had some drawbacks. It was found that the amount of secondary emission current from the plate varied unpredictably from tube to tube, and also within a single tube over its operating life; eventually it would stop oscillating. When replacing the tube, several might have to be tried to find one that would oscillate in a circuit. In addition, since dynatron oscillations were a source of instability in amplifiers, the tetrode's main application, tube manufacturers began applying a graphite coating to the plate which virtually eliminated secondary emission. By 1945 the use of the dynatron circuit was declining. In an electron tube, when electrons emitted by the cathode strike the plate, they can knock other electrons out of the surface of the metal, an effect called "secondary emission". In a normal tetrode amplifier this is an unwanted effect, and the screen grid next to the plate is biased at a lower potential than the plate, so these "secondary electrons" are repelled and return to the plate due to its positive charge. However, if the screen grid is operated at a higher potential than the plate, the secondary electrons will be attracted to it, and return to ground through the screen grid supply. This represents a current of electrons "I"G2 "away" from the plate, which reduces the net plate current "I"P below the cathode current "I"C Higher plate voltage causes the primary electrons to hit the plate with more energy, releasing more secondary electrons. Therefore, starting at the voltage at which the primary electrons have enough energy to cause secondary emission, around "V"P = 10V, there is an operating region "(grey)" in which an increase in plate voltage causes more electrons to leave the plate than the additional electrons arriving at the plate, and therefore a net reduction in plate current. Since in this region an increase in plate voltage causes a decrease in plate current, the AC plate resistance, that is the differential output resistance of the tube, is negative: As with other negative differential resistance devices like the tunnel diode, this negative resistance can be used to create an oscillator. A parallel tuned circuit is connected in the plate circuit of the tetrode. The circuit will oscillate if the magnitude of the negative plate resistance is less than the parallel resistance R of the tuned circuit, including any load connected to the oscillator. The frequency of oscillation is close to the resonant frequency of the tuned circuit. As can be seen from the graphs, for dynatron operation the screen grid had to be biased at a considerably higher voltage than the plate; at least twice the plate voltage.