Thomson's experiment

Portrait of J. J. Thomson — J. J. Thomson determined the charge-to-mass ratio of the electron in 1897.

A set of particles that have mass and charge is deflected by an electric or a magnetic field.

When a very high voltage is applied between two metal plates, the atoms of the metal of the negative plate lose electrons, which fly through the vacuum towards the positively charged plate. If, along their path, there is a voltage difference between two other plates (upper figure) or a magnetic field generated by a powerful magnet (lower figure), the electrons are deflected, which proves that they have mass and charge.

Cathode-ray tube: the electrons are deflected by an electric field (top) and by a magnetic field (bottom)

Two metal plates (electrodes: cathode and anode) are placed at the ends of an evacuated glass tube. If a high enough voltage is applied between them, a discharge takes place. If a zinc sulfide surface is placed along the axis of the discharge, a luminous ray appears on it (cathode rays). This ray is made of negatively charged particles, because it bends towards the positive pole (the anode in the upper figure) in an electric field. The particles are electrons, which have a certain mass and a negative charge. By applying electric and magnetic fields to the cathode rays, Thomson was able to measure the mass-to-charge ratio of the electron. This is the principle on which conventional television screens worked.

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