How many electrodes are in the tetrode?

In an electron gun,the control grid is given a negative potential relative to the cathode in order to:

In an amplifier,the load resistance ${R_L}$ is equal to the plate resistance ${r_p}$. The voltage amplification is equal to:

Before the saturation state of a diode,at plate voltages of $400 \, V$ and $200 \, V$,the currents are $i_1$ and $i_2$ respectively. The ratio $i_1/i_2$ will be:

In the figure,four plate characteristics of a triode at different grid voltages are shown. The difference between successive grid voltages is $1 \ V$. Which curve will have the maximum grid voltage and what is its value?

$A$ triode has a mutual conductance of $2 \times 10^{-3} \text{ mho}$ and an amplification factor of $50$. The anode is connected through a resistance of $25 \times 10^3 \, \Omega$ to a $250 \, \text{V}$ supply. The voltage gain of this amplifier is