On the left a diode, two element tube, is shown with a battery powering the filament. The negative end of the battery is connected to circuit common. Also connected to circuit common is the negative end of a higher voltage battery. The positive end is connected to one end of a resistor. the other end of this resistor is connected to the plate of the tube. This circuit is labeled heavy conduction. On the right is an almost identical circuit. The one difference is that the higher voltage battery has its positive end connected to circuit common and the negative end is connected to the resistor. This circuit is labeled no conduction.