Discovery of Electrons

1897 Discovery of Electrons

In the early 18th century, people passed electricity through gases at reduced pressure for many investigations. By about 1870, these investigation began to produce significant results. People detected that there were "rays" coming off from the cathode (i.e. the negative electrode).

Identification of Electrons

Speed of Electrons:

In 1897, Thomson measured the speed and the charge to mass ratio (e/m) called the specific charge, of cathode rays.

He proved that the cathode rays were not [electromagnetic waves] based on the fact that the speeds of the rays (3 x 10⁷ ms^-1) were typically one-tenth that of light (3 x 10⁸ ms^-1).

Deflection of Electrons:

Also, the cathode rays could be deflected by electric and magnetic fields whereas electromagnetic waves should not be deflected by these fields.

Thomson tried different cathode materials such as aluminium, copper, tin, platinum. He also used different gases in the tube such as air, hydrogen, carbon dioxide.

Diagram showing deflection of electron beam

All the figures on velocity and deflection remained the same. He was then convinced that there must be a fundamental particle( smaller than the atom) which carried negative charge.

In 1899, Thomson found that the charge of the negative particles was the same in magnitude as that carried by a hydrogen [ion], and that the mass of such a negatively charged particle was very small, compared to that of hydrogen atom. These particles became known as electrons. We now know that the mass of an electron is 1/1837 of the mass of the hydrogen atom.

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