Discovery of Electron (Cathode Rays)

The electron was the first subatomic particle to be discovered, fundamentally overturning the idea that atoms were indivisible. In the late 19th century, physicists were experimenting with evacuated glass tubes containing metal electrodes. When a high voltage was applied, a mysterious 'ray' travelled from the negative electrode (cathode) to the positive electrode (anode). These were called cathode rays, and their true nature — streams of negatively charged particles — was conclusively demonstrated by J. J. Thomson in 1897. Thomson's genius was to subject cathode rays to both electric and magnetic fields simultaneously, allowing him to measure the ratio of the particle's charge to its mass (e/m). He found this ratio to be the same regardless of the cathode material or the residual gas in the tube, proving that these particles were a universal constituent of all matter. This was the first experimental evidence that atoms contain smaller components. The electron's discovery launched the age of subatomic physics. Thomson's measured value of e/m ≈ 1.76 × 10¹¹ C kg⁻¹ is remarkably close to the modern accepted value. Later, Robert Millikan's oil-drop experiment (1909) measured the electron's charge e directly, allowing the mass of the electron to be calculated as approximately 9.109 × 10⁻³¹ kg — nearly 1836 times lighter than a hydrogen atom.

Key takeaways

The Apparatus
Deflection by Electric Field
Deflection by Magnetic Field
Velocity Selector — Balancing the Fields
Measuring e/m — Electric Deflection Alone
Key Result

The Apparatus

Thomson used a highly evacuated glass tube with a heated cathode (negative electrode) and a perforated anode (positive electrode). A narrow beam of cathode rays passed through the anode hole and struck a fluorescent screen at the far end, producing a bright spot. Two parallel electric-field plates and external magnetic-field coils were positioned around the beam path.

Deflection by Electric Field

When an electric field E was applied between the plates (plate separation d, voltage V so E = V/d), the beam deflected toward the positive plate. This confirmed the rays carried negative charge. The electric force on a particle of charge e is F_E = eE, giving it a transverse acceleration a = eE/m.

Deflection by Magnetic Field

A magnetic field B applied perpendicular to the beam exerts a Lorentz force F_B = evB on a particle moving with velocity v. This curves the beam in the opposite direction to the electric field. By noting the direction of both deflections, Thomson confirmed the charge was negative.

Velocity Selector — Balancing the Fields

Thomson's key trick was to apply E and B simultaneously and adjust their magnitudes until the beam travelled perfectly straight (no net deflection). At this balance point, the electric and magnetic forces are equal: eE = evB. Solving for the particle velocity: v = E/B. This gave a direct measurement of the speed of the particles without needing to know e or m separately.

Measuring e/m — Electric Deflection Alone

Thomson then switched off the magnetic field and measured the deflection y of the spot on the screen caused by E alone. For a particle of mass m and charge e travelling length L through the plates and then a further distance D to the screen, the transverse deflection is y = (eE/m) × (L/v)² / 2 + (eE/m)(L/v)(D/v). Since v is already known from step 4, all quantities except e/m are measurable, so e/m can be calculated directly.

Key Result

Thomson obtained e/m ≈ 1.7 × 10¹¹ C kg⁻¹ for cathode ray particles — about 1836 times larger than the e/m ratio for a hydrogen ion (proton). Since e was later found to be the same as the proton charge, the cathode ray particle (electron) must be ~1836 times lighter than a proton. Thomson also showed the value was independent of electrode material and gas type, proving electrons are universal constituents of atoms.

The Key Equation

\frac{e}{m} = \frac{E}{B^2 y} \cdot \frac{1}{\ell}

e — Charge of electron (C (coulombs)): The magnitude of the electron's charge; e = 1.602 × 10⁻¹⁹ C
m — Mass of electron (kg): Rest mass of the electron; m = 9.109 × 10⁻³¹ kg
E — Electric field strength (V m⁻¹): The uniform field between the deflecting plates, E = V/d
B — Magnetic flux density (T (tesla)): The external magnetic field applied perpendicular to the beam
y — Transverse deflection (m): Measured displacement of the beam spot on the screen
\ell — Effective plate length (m): Length of the electric field region through which the beam passes

Worked Example

Problem. In a cathode ray tube experiment, an electric field of E = 2.0 × 10⁴ V m⁻¹ and a magnetic field of B = 5.0 × 10⁻³ T are balanced so the beam is undeflected. The electric field is then switched off and the beam deflects by y = 0.030 m over an effective plate length of ℓ = 0.050 m. Calculate: (a) the speed of the electrons, and (b) the charge-to-mass ratio e/m.

v = 4.0 \times 10^{6}\text{ m s}^{-1},\quad \frac{e}{m} = \frac{2yv^{2}}{E\ell^{2}} \approx 1.92 \times 10^{10}\text{ C kg}^{-1}

Answer. Speed v = 4.0 × 10⁶ m s⁻¹; charge-to-mass ratio e/m ≈ 1.92 × 10¹⁰ C kg⁻¹ (for the simplified in-plate deflection only).

Figure

Diagram

Origins · 1 minJoseph John Thomson and how this was discovered

Joseph John Thomson

1856–1940 · British

Thomson discovered the electron in 1897 through his cathode ray tube experiments at the Cavendish Laboratory, Cambridge. He measured the charge-to-mass ratio (e/m ≈ 1.76 × 10¹¹ C kg⁻¹) and showed it was a universal property of matter, independent of electrode material or gas type. He was awarded the Nobel Prize in Physics in 1906. Thomson's seven doctoral students went on to win Nobel Prizes themselves, including Ernest Rutherford. He also served as director of the Cavendish Laboratory for 34 years and was knighted in 1908.

Discovery of Nucleus (Rutherford) — Rutherford's gold-foil experiment was the next landmark in probing atomic structure after Thomson discovered electrons, revealing that positive charge is concentrated in a tiny nucleus.
Neutrons & Protons — The electron was the first subatomic particle found; protons and neutrons are the other fundamental nuclear particles that complete the picture of atomic composition.
Atomic Number & Mass Number — Atomic number Z equals the number of protons (and electrons in a neutral atom), directly connecting to the electron count established since Thomson's discovery.
Isotopes, Isobars, Isotones — Understanding isotopes requires knowing the numbers of protons, neutrons and electrons — subatomic particles whose existence was established starting with the electron.
Mass Spectrometry & Isotopes — Mass spectrometers use the same principle as Thomson's experiment — charged particles deflected by magnetic and electric fields — to separate ions by their mass-to-charge ratio.
Thomson's Model — After discovering the electron, Thomson proposed the 'plum pudding' atomic model to explain how electrons are embedded in a diffuse positive charge — the direct conceptual follow-up to this experiment.
Rutherford's Model — Rutherford's nuclear model replaced Thomson's model, but both models rest on the existence and properties of the electron established by Thomson's cathode ray work.
Bohr's Model (Qualitative) — Bohr's model quantised the energy levels of electrons orbiting the nucleus — the very electrons whose existence Thomson first demonstrated.
Postulates of Bohr Theory — Bohr's postulates describe the allowed orbits for electrons; understanding why electrons matter in atomic theory starts with their discovery in cathode ray experiments.
Radii & Velocity Derivations — The derivation of Bohr orbit radii and electron velocities uses the electron mass m — a quantity whose determination traces back to Thomson's e/m measurement combined with Millikan's charge measurement.