In 1897, an English Physicist J.J. Thomson studied the effect of electric field only cathode rays.

The discharge tube used by Sir J.J. Thomson consisted the following parts:

  • A cathode(C)
  • A cylindrical metal disc having a fine hole at its center, acts as anode (A). This accelerates the particles of the cathode rays.
  • Another metal disc (D) having a fine hole at its center in line with the hole in the anode.
  • The cathode rays (consisting of electrons) pass through these holes and strike the fluorescent screen at the point E.
  • Two flat plates P1 and P2, which can be connected to a source of high voltage.
  • An electromagnet generating field opposite to the field generated by the plates P1 and P2.

In the absence of any electrical or magnetic field, the cathode rays strike the fluorescent screen at point Those, and can be seen as a bright spot there. When a high electric field is applied across the plates P1 and P2 the spot only the screen moves towards the positively charged plate.

The bending of cathode rays towards the positive plate showed that the cathode rays were constituted of negatively charged particles. The deflection suffered by the beam of cathode rays (or beam of electrons) in the Thomson’s experiment depends upon the strength of the electric field applied across the electrodes.


The charge-to mass ratio of an electron

J.J. Thomson studied the combined effect of electric and magnetic fields on the cathode rays. The two fields were applied in such a way that the deflection ue to the electric field is cancelled out by the deflection due to the magnetic field, so that the net deflection was zero.

From the strengths of electric and magnetic fields are required to balance the deflection, the ratio of charge to mass (e/ m) of the particles constituting these rays were found out. It was found that the ratio of charge (e) to mass (m) of such particles is same, and does not depend only the nature of gas used in the discharge tube.

The e/ make value for the particles in the cathode rays was found to have a constant value of 1.76 × 1011 C / kg, i.e.,


e/m= 1.76 × 1011 C / kg= constant

From this experiment, Thomson concluded that all atoms contained the same type of negative particles, called electrons.

The charge and mass of an electron

In 1909, an American physicist, Robert A. Millikan accurately determined the charge on an electron by performing his well-known “oil-drop” experiment. He studied the motion of charged oil-drops in the space between two charged metal plates. From the experiment, he found that the charge on the oil drops was always a multiple of an elementary charge of 1.6× 10-19 coulombs. From these results and those obtained from the studies of cathode rays, it was found that the charge only the cathode ray particle was one unit of elementary charge.