J.J. Thomson's cathode-ray tube experiment demonstrated that

Thomson deflected cathode rays with crossed electric and magnetic fields and measured their specific charge, $\dfrac{e}{m} \approx 1.76 \times 10^{11}$ C/kg.
He found that this value is the same whatever gas is used in the tube and whatever the cathode material, so cathode rays consist of identical particles (electrons) present in all atoms. They are not negatively charged ions.
The value was about 1837 times larger than the e/m of the hydrogen ion (proton), since the electron has the same magnitude of charge but much smaller mass:
$\left(\dfrac{e}{m}\right)_{electron} \gg \left(\dfrac{e}{m}\right)_{proton}$, with $\left(\dfrac{e}{m}\right)_{proton} = \dfrac{1}{1837}\left(\dfrac{e}{m}\right)_{electron}$
The nucleus was discovered later by Rutherford's alpha scattering experiment.
So the experiment demonstrated that the e/m of electrons is much greater than the e/m of protons.