In normal applications, the anode voltage was about 150 V, while that of the screen-grid was about 60 V (Thrower p 183). As the screen grid is positive with respect to the cathode, it collects a certain fraction (perhaps a quarter) of the electrons which would otherwise pass from the grid region to the anode. This causes current to flow in the screen grid circuit. Usually, the screen current due to this cause is small, and of little interest. However, if the anode voltage should be below that of the screen, the screen grid can also collect secondary electrons ejected from the anode by the impact of the energetic primary electrons. Both effects tend to reduce the anode current. If the anode voltage is increased from a low value, with the screen grid at its normal operating voltage (60V, say) the anode current initially increases rapidly because more of those electrons which pass through the screen-grid are collected by the anode rather than passing back to the screen grid. This part of the tetrode anode characteristic resembles the corresponding part of that of a triode or pentode. However, when the anode voltage is increased further, the electrons arriving at the anode have sufficient energy to cause copious secondary emission, and many of these secondary electrons will be captured by the screen, which is at a higher positive voltage than the anode. This causes the anode current to fall rather than increase when the anode voltage is increased. In some cases the anode current can actually become negative (current flows out of the anode); this is possible since each primary electron may produce more than one secondary. Falling positive anode current accompanied by rising anode voltage gives the anode characteristic a region of negative slope, and this corresponds to a negative resistance which can cause instability in certain circuits. In a higher range of anode voltage, the anode voltage sufficiently exceeds that of the screen for an increasing proportion of the secondary electrons to be attracted back to the anode, so the anode current increases once more, and the slope of the anode characteristic becomes positive again. In a yet higher range of anode voltages, the anode current becomes substantially constant, since all of the secondary electrons now return to the anode, and the main control of current through the tube is the voltage of the control grid. This is the normal operating mode of the tube.