Why is a vacuum tube called a valve in electronics?

The variation of anode current $(I_p)$ in a triode corresponding to a change in grid potential $(V_g)$ at three different values of the plate potential $(V_p)$ is shown in the diagram. The mutual conductance $(g_m)$ of the triode is.........$ m \text{ mho}$.

The plate current in a triode is given by ${I_p} = 0.004 ({V_p} + 10{V_g})^{3/2} \text{ mA}$,where ${I_p}, {V_p}$ and ${V_g}$ are the values of plate current,plate voltage,and grid voltage,respectively. What are the triode parameters $\mu$,${r_p}$ and ${g_m}$ for the operating point at ${V_p} = 120 \text{ V}$ and ${V_g} = -2 \text{ V}$?

For a given plate-voltage,the plate current in a triode is maximum when the potential of

Which of the following are triode constants?

The relation between dynamic plate resistance $(r_p)$ of a vacuum diode and plate current $(I_p)$ in the space charge limited region is: