Electrons were discovered through a series of experiments involving cathode ray discharge tubes.
J. J. Thomson – Cathode ray
He constructed a glass tube that was partially evacuated, i.e. much of the air was pumped out of the tube. Then he applied a high electrical voltage between two electrodes at either end of the tube. He detected that a stream of the particle (ray) was coming out from the negatively charged electrode (cathode) to the positively charged electrode (anode). This ray is called a cathode ray, and the whole construction is called a cathode ray tube.
- They travel in straight lines.
- They are independent of the material composition of the cathode.
- Applying an electric field in the cathode ray path deflects the ray towards the positively charged plate. Hence, the cathode ray consists of negatively charged particles.
Millikan Oil Drop Experiment
R. Millikan measured the charge of an electron using negatively charged oil droplets. The measured charge (e) of an electron is −1.60×10−19 Coulombs. Using the electron’s measured charge, we can calculate the mass of the electron from the e/m ratio given by J. J. Thomson’s cathode ray experiment.