In $1897$,$J$.$J$. Thomson discovered which particle?

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

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

Cathode rays enter into a uniform magnetic field perpendicular to the direction of the field. In the magnetic field,their path will be:

$(a)$ 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?

The order of the $q/m$ ratio for a proton,$\alpha$-particle,and electron is: