The grid in a high-vacuume triode is usually kept negatively charged with respect to the cathode so that the electrons may be
controlled or modulated in their flow. This negative charge on the grid is referred to as the grid bias voltage. The purpose of applying a negative bias to the grid is to create an electric field that repels and controls the movement of electrons emitted from the cathode.
To maintain this negative charge on the grid, a power supply is connected to the grid terminal of the triode. The power supply provides a DC voltage that is lower than the cathode voltage, creating a potential difference between the grid and the cathode.
Now, to answer your question, the current flowing in the grid circuit is typically very low and can be negligible compared to the current flowing in the anode (plate) circuit. This is due to the high resistance present between the grid and the power supply. Therefore, the electrons emitted from the cathode will not be significantly affected by the small current flowing through the grid circuit.
To ensure the grid remains negatively charged, the resistance in the grid circuit needs to be sufficiently high. This can be achieved by using resistors or high impedance components in the grid circuit to limit the current flowing through it.
In summary, the negative charge on the grid of a high-vacuum triode is maintained by applying a negative bias voltage from a power supply. The current flowing in the grid circuit is usually very low due to the high resistance, ensuring that the electrons emitted from the cathode are not substantially affected.