English: Graph of plate (anode) and screen grid current vs plate voltage in a UY224 1920s era tetrode vacuum tube, showing region of negative resistance (grey) used in dynatron oscillator circuit.

I_P = Plate (anode) current

I_G2 = Screen grid current

I_C = Cathode current is equal to the sum of I_P and I_G2

Control grid potential V_G1 = −1.5 V

Screen grid potential V_G2 = 75 V

When electrons from the cathode hit the plate, they knock secondary electrons out of the metal, a process called secondary emission. If the screen grid is made positive with respect to the plate, the secondary electrons will be attracted to it, representing a current away from the plate. For a certain plate voltage range (grey) an increase in plate voltage will cause a greater increase in secondary electrons than primary electrons, so the plate current will decrease. Thus the differential plate resistance is negative:

r_P = ΔV_P / ΔI_P < 0

This negatve resistance was used in the dynatron oscillator circuit to cancel the resistance in a tuned circuit, creating oscillations. The dynatron oscillator was used in some instruments in the 1930s and 40s. In modern tetrodes the surface of the plate is treated to greatly reduce secondary emission, so they don't have negative resistance. Note:

• Due to the fact that the screen grid screens out the electric field of the plate, the cathode current I_C is almost constant with plate voltage. So the plate and screen currents are almost mirror images of each other
• Within the range V_P = 20 - 60 V each electron striking the plate actually knocks out more than one secondary electron, so the plate current actually reverses - more electrons leave the plate than enter it