Here's how:
1. Cathode Ray Tubes: Thomson used cathode ray tubes, which are glass tubes with electrodes at either end. When a high voltage is applied, a beam of particles (later identified as electrons) is emitted from the cathode and travels towards the anode.
2. Magnetic and Electric Fields: Thomson applied both magnetic and electric fields to the beam of particles. By carefully adjusting the fields, he could deflect the beam in a known way.
3. Measuring Deflection: He measured how much the beam deflected in both the electric and magnetic fields.
4. Calculating e/m: Using the measurements of deflection and the known strengths of the fields, he calculated the charge-to-mass ratio (e/m) of the electron. This value was surprisingly high, indicating that the electron had a very small mass.
It wasn't until later, in 1911, that Robert Millikan's oil drop experiment determined the charge of the electron. This allowed scientists to then calculate the mass of the electron using the charge-to-mass ratio established by Thomson.
