In the late 19th century, physicists were puzzled by mysterious rays observed in vacuum tubes. When electrical current passed through these evacuated glass tubes, a glowing beam appeared to travel from the cathode, or negative electrode, to the anode, or positive electrode. These cathode rays created a bright spot on the fluorescent screen at the end of the tube, but their true nature remained unknown.
J.J. Thomson revolutionized cathode ray research with his ingenious experimental design. His apparatus featured an evacuated glass tube containing a cathode and anode to generate the ray beam. Crucially, he added parallel electric plates and magnetic field coils that could deflect the beam in controlled ways. A fluorescent screen at the end detected the beam position, allowing Thomson to measure deflections precisely and determine the fundamental properties of these mysterious rays.
Thomson's first crucial observation was that cathode rays deflect in an electric field. When he applied voltage to the parallel plates, the beam curved toward the positive plate, proving the rays carry negative charge. The deflection angle depends on the electric field strength, particle velocity, and the charge-to-mass ratio. This relationship, expressed as theta equals e-E-L over m-v-squared, became fundamental to determining the particle properties.
When Thomson applied a magnetic field perpendicular to the cathode ray beam, he observed a different type of deflection. The magnetic field caused the charged particles to follow a circular arc trajectory. The Lorentz force, equal to q-v-B, acts perpendicular to both velocity and magnetic field. The radius of curvature is given by r equals m-v over q-B, providing another relationship between particle properties and measurable deflection.
Thomson's genius lay in combining both electric and magnetic field measurements. First, he balanced the electric and magnetic forces to determine the particle velocity using v equals E over B. Then, using magnetic deflection alone, he calculated the charge-to-mass ratio as e over m equals v over r-B. His revolutionary result of 1.76 times 10 to the 11th coulombs per kilogram was nearly 2000 times larger than any known ion, proving these cathode ray particles were a new fundamental constituent of matter - the electron.