The number of secondary electrons emitted per number of primary electrons depends on:

$(a)$ 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?

Positive rays consist of

Which scientist invented cathode rays?

In an electron gun, electrons are accelerated by a potential difference $V$. If $e$ is the charge and $m$ is the mass of an electron, then the maximum velocity of these electrons will be

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