Thomson's Experiment

Thomson's Experiment

  • One of the Scientists that worked with Cathode Ray Tubes was an experimentalist named J. J. Thomson.
  • He was the one who discovered that the Cathode Rays were deflected by electric fields.
  • After that experiment, he worked on another experiment with Cathode Ray Tubes.
  • The aim of the experiment was to measure the Charge to Mass ratio of the Cathode Rays.
  • It was set up as follows:
    • A CRT was built with two parallel electric plates in one axis
    • A magnetic field was produced by current carrying coils in the other axis (perpendicular to the electric plates).
    • The Cathode Rays were passed through two thin collimator slits to produce a thin beam rather than a Ray.
    • This was aimed at a fluorescent screen to produce a dot.
  • The experiment was as follows:
  • Stage 1
    • The electron gun was initially fired without any electrical input (no E or B).
    • This position of the dot was noted on the screen.
    • Then both fields were turned on.
    • The strengths of the fields were then altered so that it passed through undeflected.
      • This means that the dot was at its initial position without the fields on.
      • This was to equate the Magnetic force and the Electrical force.
\begin{equation} F_{magnetic} = F_{electric} \end{equation}
\begin{equation} qvB = qE \end{equation}
\begin{align} v = \frac{E}{B} \end{align}
  • Stage 2
    • The Electric field was then turned off.
      • This meant that there was now a deflection, but it was only due to the magnetic field.
    • Thompson then measured the radius of this deflection.
\begin{equation} F_{magnetic} = F_{centripetal} \end{equation}
\begin{align} qvB = \frac{mv^2}{r} \end{align}
\begin{align} \frac{q}{m} = \frac{v}{Br} \end{align}

Substitute (3) in

\begin{align} \frac{q}{m} = \frac{E}{B^2r} \end{align}

And so the charge to mass ratio could be found.


  • Thomson found that the ratio of the Cathode Ray particles was 1800 times greater than that of a Hydrogen ion
    • He then decided that its mass was 1800 times smaller rather than having a charge 1800 times greater through a series of other experiments.
    • Which meant that it was a sub-atomic particle
  • Since the ray was the same regardless of the materials used for the Cathode, he proved that these Cathode Ray particles were a constituent of every atom.
    • Later, these would be known as electrons.