Thomson's experiment revealed the electron as a universal subatomic particle

Turning Points in Physics | AQA A-Level Physics

What Thomson measured

J.J. Thomson used the crossed-fields method to measure the specific chargeThe charge-to-mass ratio of a particle, $e/m_e$, measured in C kg$^{-1}$. of cathode rays.
He found a value of approximately $1.76 \times 10^{11}$ C kg$^{-1}$.
The key part is the comparison: the specific charge of a hydrogen ion (the smallest known charged particle at the time) is $9.58 \times 10^{7}$ C kg$^{-1}$.
The electron's specific charge is therefore about 1800 times larger than that of the hydrogen ion.

Why this was significant

Since the electron and the hydrogen ion carry the same magnitude of charge ($e$), the only way the electron's specific charge could be 1800 times larger is if its mass is 1800 times smaller.
This meant the electron was far lighter than any atom, proving that atoms are not indivisible. There must be particles smaller than atoms.
Thomson found the same specific charge regardless of what gas was in the discharge tube or what metal was used for the cathode.
This universality showed that the electron is a fundamental constituent of all matter, not something unique to a particular element.

Limitations of the experiment

Thomson could only measure $e/m_e$, not $e$ or $m_e$ individually. It took Millikan's oil drop experiment (1909) to measure $e$ on its own.
Once $e$ was known, $m_e$ could be calculated: $m_e = e \,/\, (e/m_e) = 9.11 \times 10^{-31}$ kg.

Common Mistake

Students sometimes say Thomson "discovered the electron's charge." He did not. He measured the specific charge $e/m_e$. Millikan later measured $e$ independently. Make sure you distinguish between these two experiments.