Cathode rays

Properties of Cathode rays

  1. They travel in straight lines from the negative pole (cathode).
  2. They produce fluorescence in the glass walls of the discharge tube.
  3. They cast shadows if some target is placed in their path.
  4. They can produce mechanical motion, e.g., they cause a light pedal wheel placed in their path to rotate.
  5. They possess heating effect and can heat thin metal filaments to incandescence.
  6. They are deflected from their rectilinear path by electrostatic and magnetic fields and behave in the manner of a stream of negatively charged particles.
  7. They can impart negative charge to objects in their paths.
  8. They can cause ionization in gases.


Thomson studied the deflection of these rays under the influence of magnetic and electrically charged plates and by counterbalancing the effect of the electrical and magnetic field on the cathode rays, Thomson was able to find out the ratio of the charge to mass (e/m) of the cathode particle. In S. I. units, the value of (e/m) of cathode particles is –1.76 × 108 C/g. Thomson showed that the value of e/m of cathode particles was same regardless of both, the gas and the metal electrode. This proved that the particles making up the cathode rays were all identical and were constituent parts of all atoms. Dutch Physicist, H.A Lorentz named them Electrons. Later on the charge on electron was found to be 1.6×10-19C by R.A Milliken, and hence mass of electron was found to be 9.1×10-31 Kg or 1/1837th of Hydrogen atom.