Discovery of Proton

Much before the discovery of electron, Eugen Goldstein (in 1886) performed an experiment using a perforated cathode (a cathode having holes in it) in the discharge tube filled with air at a very low pressure. When a high voltage was applied across the electrodes in the discharge tube, a faint red glow was observed behind the perforated cathode.

This glow was due to another kind of rays flowing in a direction opposite to that of the cathode rays. These rays were called as anode rays or positive rays. These were positively charged and were also called canal rays because they passed through the holes or the canals present in the perforated cathode.

The following observations were made about anode rays (canal rays):

  • Like cathode rays, the anode rays also travel in straight lines.

  • The particles constituting anode rays carry mass and have kinetic energy.

  • The particles constituting canal rays are much heavier than electrons and carry positive charges.

  • The positive charge on the particles was whole number multiples of the amount of charge present on the electron.

  • The nature and the type of the particles constituting the anode rays were dependent on the gas present in the discharge tube.

The origin of anode rays can be explained in terms of interaction of the cathode rays with the gas present in the vacuum tube. The electrons emitted from the cathode collide with the neutral atoms of the gas present in the tube and remove one or more electrons present in them. This leaves behind positive charged particles which travel towards the cathode. When the cathode ray tube contained hydrogen gas, the particles of the canal rays obtained were the lightest and their charge to mass ratio (e/m ratio) was the highest.

Rutherford showed that these particles were identical to the hydrogen ion (hydrogen atom from which one electron has been removed). These particles were named as protons and were shown to be present in all matter. Thus, the experiments by Thomson and Goldstein had shown that an atom contains two types of particle which are oppositely charged and an atom is electrically neutral.