The negative terminal of the b-battery is connected to the filament.
Now, suppose that the b-battery takes away 100 of these each second.
Suppose that we take the same tube and use a b-battery of lower voltage.
This new battery we shall call the “plate battery” or the “b-battery.”
Of course, as long as the switch in the b-battery is open no current can flow.
The b-battery attracts electrons to the plate and so reduces the crowd in the tube.
We might say that changing the b-battery shifted the curve with reference to the axis of zero volts on the grid.
The b-battery will want to take in electrons at its positive terminal and to send them out at its negative terminal.
In the plate-filament circuit we connect the b-battery and a switch, S, and another coil.
Of course, at that same instant an equal number leave the other plate and start off toward the b-battery and the filament.