The color of the positive column in a gas discharge tube depends on:

The ratio of thermionic currents $(I/I_0)$ for a metal when the temperature is slowly increased from $T_0$ to $T$ is shown in the figure. ($I$ and $I_0$ are currents at $T$ and $T_0$ respectively). Which curve correctly represents this relationship?

Positive rays were discovered by

The minimum value of electric field required to pull out electrons from a metal is approximately . . . . . . $V$/cm.

In a discharge tube at $0.02\, mm$ pressure,there is a formation of
[$AIPMT$ $1996$]

Which one is the correct relation for thermionic emission?