Discovery of the Electron
In the late nineteenth century, scientists were puzzled by a mysterious glow that appeared inside sealed glass tubes when electric current was passed through them at very low pressure. These glowing streams, travelling from the negative electrode (cathode) to the positive electrode (anode), were called cathode rays. Nobody agreed on what they actually were — some thought they were waves like light, while others suspected they were streams of charged particles. The answer would overturn centuries of belief that the atom was the smallest possible unit of matter. In 1897, British physicist J.J. Thomson performed a landmark series of experiments at the Cavendish Laboratory in Cambridge. By applying both electric and magnetic fields to cathode rays, he was able to measure the ratio of the electric charge to the mass of each particle — a quantity called the specific charge, written as e/m. He discovered this ratio was the same no matter what metal the cathode was made of or what gas was left in the tube. This meant the particles were not atoms or molecules, but something far smaller and more fundamental: subatomic particles, which we now call electrons. Thomson's discovery shattered the idea of the indivisible atom and launched the era of subatomic physics. It earned him the Nobel Prize in Physics in 1906. The measured specific charge of the electron, approximately 1.76 × 10¹¹ C kg⁻¹, remains one of the most precisely known physical constants and forms the foundation for understanding electricity, chemistry, and quantum mechanics.
- Setting Up the Cathode Ray Tube
- Applying an Electric Field
- Applying a Magnetic Field
- Measuring the Deflection to Find e/m
- Universality of the Result
Setting Up the Cathode Ray Tube
Thomson used an evacuated glass tube with a metal cathode at one end and an anode with a small hole at the other. A high voltage (several kilovolts) was applied across the electrodes. Electrons were knocked off the cathode surface and accelerated towards the anode, passing through the hole to form a narrow beam. A fluorescent coating at the far end of the tube made the beam visible as a bright spot — essentially the same principle as an old television set.
Applying an Electric Field
Thomson introduced two parallel metal plates inside the tube and applied a voltage across them, creating a uniform electric field E perpendicular to the beam. The beam deflected towards the positive plate, confirming the particles carried negative charge. The electric force on each particle is F = eE, where e is the magnitude of the electron's charge. This deflection alone could not give e and m separately — it only told Thomson the particles were negatively charged.
Applying a Magnetic Field
Thomson then added a magnetic field B, also perpendicular to the beam but at right angles to the electric field. By adjusting E and B until the beam was perfectly straight (undeflected), he knew the electric force and magnetic force exactly cancelled: eE = evB, where v is the speed of the particles. This gives v = E/B directly — a velocity selector that required no knowledge of e or m.
Measuring the Deflection to Find e/m
With the magnetic field alone switched on, the beam curved in a circular arc of radius r. For circular motion, the magnetic force provides the centripetal force: evB = mv²/r. Rearranging and substituting v = E/B gives the specific charge: e/m = E / (B²r). Thomson measured E, B, and r geometrically from the deflection of the spot on the screen. This elegantly yielded e/m without needing to know e or m individually. He obtained e/m ≈ 1.7 × 10¹¹ C kg⁻¹, about 1800 times larger than for the hydrogen ion, implying the electron is far lighter than any atom.
Universality of the Result
Crucially, Thomson repeated the experiment with different cathode materials (platinum, aluminium, iron) and different residual gases in the tube. Every time, he obtained the same value of e/m. This consistency proved that these particles were a universal constituent of all matter — not specific to any one element. Combined with Robert Millikan's 1909 oil-drop experiment that measured e directly (≈ 1.6 × 10⁻¹⁹ C), the mass of the electron could be calculated as m = e/(e/m) ≈ 9.11 × 10⁻³¹ kg.
The Key Equation
- e — Elementary charge (C): The magnitude of the charge on one electron, approximately 1.60 × 10⁻¹⁹ C.
- m — Electron mass (kg): The rest mass of one electron, approximately 9.11 × 10⁻³¹ kg.
- E — Electric field strength (V m⁻¹): The uniform electric field between the deflecting plates inside the cathode ray tube.
- B — Magnetic flux density (T): The strength of the magnetic field applied perpendicular to the electron beam.
- r — Radius of curvature (m): The radius of the circular arc followed by electrons under the magnetic force alone.
Worked Example
Problem. In a Thomson-style experiment, an electric field of 2.0 × 10⁴ V m⁻¹ and a magnetic field of 4.0 × 10⁻³ T are balanced so the beam is undeflected. The magnetic field alone is then applied and the beam follows a circular arc of radius 0.14 m. Calculate (a) the speed of the electrons, and (b) the specific charge e/m.
Answer. Electron speed v = 5.0 × 10⁶ m s⁻¹; specific charge e/m = 8.9 × 10⁹ C kg⁻¹ (using the given values).
Origins · 1 minJ.J. Thomson and how this was discovered
Joseph John Thomson discovered the electron in 1897 by measuring the specific charge (e/m) of cathode ray particles. He demonstrated that these particles were universal constituents of all matter — the first subatomic particles ever identified. Thomson went on to propose the 'plum pudding' model of the atom and also discovered the existence of isotopes. He was awarded the Nobel Prize in Physics in 1906. Thomson was famously humble about his own mathematical abilities as a young student, yet he led the Cavendish Laboratory to produce no fewer than seven Nobel Prize winners among his students and colleagues, including Ernest Rutherford.
Related Concepts
- The Photoelectric Effect — The photoelectric effect is another key phenomenon involving electron emission — light ejects electrons from a metal surface, and Einstein's explanation helped confirm the quantum nature of light and the particle nature of the electron.
- Millikan's Oil-Drop Experiment — Millikan's 1909 experiment measured the charge e of the electron directly, allowing the electron mass to be calculated from Thomson's e/m ratio — completing the picture of the electron as a particle.
- Models of the Atom — Thomson's discovery of the electron led directly to the first atomic model with internal structure — the 'plum pudding' model — which was later replaced by Rutherford's nuclear model.
- Magnetic Force on a Moving Charge — The deflection of the electron beam in Thomson's experiment relies on the Lorentz magnetic force F = qvB, making this a direct application of electromagnetic field theory.