To emit an electron from a metal surface, the minimum electric field required is . . . . . . .

Thermionic emission from a heated filament varies with its temperature $T$ as

An electron is:

Positive rays were discovered by

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?

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