The grid in a high-vacuum triode is usually kept negatively charged
with respect to the cathode so that the electrons may be attracted to
the anode instead of the grid (B).
I would bet [my own] money that they are not teaching you this material in
high school physics. You will never ever have any use for this information,
unless maybe you're a contestant on Jeopardy some day, and the category
for Final Jeopardy is "Electronic Small-Signal Amplification Devices in the
First Half of the Twentieth Century".
