In the late 19th century, physicists discovered mysterious radiation emanating from cathode tubes. When electric current flowed through an evacuated glass tube, strange rays traveled from the negative cathode to the positive anode. The fundamental question arose: what exactly were these cathode rays made of? This discovery would revolutionize our understanding of atomic structure.
To determine the nature of cathode rays, scientists applied an electric field perpendicular to their path using parallel plates. When the upper plate was positively charged and the lower plate negatively charged, the cathode rays deflected toward the positive plate. This deflection proved that cathode rays consist of negatively charged particles, not electromagnetic waves, as the force equals charge times electric field strength.
When a magnetic field is applied perpendicular to both the cathode ray path and electric field, the rays follow a circular trajectory. The magnetic force equals charge times velocity times magnetic field strength. By crossing electric and magnetic fields, scientists could balance the forces, making rays travel straight. This balanced condition allows direct calculation of particle velocity as electric field divided by magnetic field.
Thomson's breakthrough came from combining both electric and magnetic deflection measurements. He derived that electric deflection depends on charge times electric field times length squared, divided by twice mass times velocity squared. Magnetic deflection relates to charge times magnetic field times length times velocity over mass. Using the balanced field velocity, he eliminated velocity from the equations and calculated the charge-to-mass ratio as 1.76 times 10 to the 11th coulombs per kilogram - remarkably, 1837 times larger than hydrogen ions, indicating these particles were much lighter and more fundamental.
Thomson's cathode ray experiments led to the discovery of the electron - the first subatomic particle ever identified. These fundamental negatively charged particles have a mass 1837 times smaller than hydrogen atoms and carry a charge of negative 1.6 times 10 to the minus 19 coulombs. This discovery revolutionized atomic theory, leading from the plum pudding model to nuclear atomic models, Bohr's quantum theory, and ultimately modern quantum mechanics and the electronics revolution that shapes our world today.