The thermionic emission of electrons is due to:

Positive rays were discovered by

$(a)$ Estimate the speed with which electrons emitted from a heated emitter of an evacuated tube impinge on the collector maintained at a potential difference of $500\;V$ with respect to the emitter. Ignore the small initial speeds of the electrons. The specific charge of the electron,$i.e.$,the $e/m$ is given to be $1.76 \times 10^{11}\; C\; kg^{-1}$.
$(b)$ Use the same formula you employ in $(a)$ to obtain electron speed for a collector potential of $10\;MV$. Do you see what is wrong? In what way is the formula to be modified?

If the thermionic current density is $J$ and emitter temperature is $T$,then the curve between $\frac{J}{T^2}$ and $\frac{1}{T}$ will be

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

Assertion: In a photoemissive cell,an inert gas is used.
Reason: An inert gas in the photoemissive cell provides a greater current.