In 1897, British physicist J.J. Thomson made a groundbreaking discovery: the electron, the first known subatomic particle. He made this discovery while experimenting with cathode ray tubes—sealed glass tubes with metal electrodes at each end and low-pressure gas inside.
When high voltage was applied, a beam (later called a cathode ray) traveled from the negative electrode (cathode) to the positive one (anode). Thomson showed that this ray was made of negatively charged particles, much smaller than atoms. By measuring how the beam was deflected by electric and magnetic fields, he calculated the charge-to-mass ratio and concluded that these particles were universal components of atoms.
Thomson’s discovery shattered the belief that atoms were indivisible and marked the birth of modern atomic theory. It also led to the development of early atomic models and set the stage for future discoveries in nuclear and quantum physics. For this work, Thomson received the Nobel Prize in Physics in 1906.