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J.J. Thomson's experiment to determine the sp. charge of an electron

J.J. Thomson's experiment to determine the specific charge of an electron.

The specific charge is simply the ratio of charge of an electron to its mass.
The value of sp. charge of an electron is found 1.70 x 1011C/Kg.

fig. Thomson's apparatus for determination of specific charge of an electron.


The specific charge of an electron can be determined when the electron moves in both magnetic and electric fields which are mutually perpendicular to each other, so the net force on the electron is made zero. In this situation, the direction of motion of electrons remains perpendicular to both the fields.


The experiment setup consists of an evacuated glass tube provided with the cylindrical anode (A) with a hole at its center and the cathode (C) in the form of filament which is maintained at the potential difference of V volts. The electrons are emitted from the filament cathode when suitable current is passed, through the hole H of the anode. The electron beam then travels straight along the horizontal axis of the tube and strikes in the middle of the fluorescent screen at the other end of the tube at point O.

Working Principle.

When a sufficient amount of potential difference (V) is applied between two electrodes, electrons from the cathode accelerates with velocity (v) then,

When the charged particle moves under the action of electric field strength (E) it moves towards the +ve plate and finally incident at a point O, at the fluorescent screen. In the case of applying the magnetic field only, the charged particle deviates and finally incident at 02 as shown in the figure. If both magnetic and electric fields are applied and their magnitude and direction are adjusted so that the charged particles move without deviation. At this condition charge particle incident at point O at the fluorescent screen,

We have, 

Where B is magnetic field strength. In this case, the direction of the magnetic field is perpendicular to the direction of the motion of the charged particle.

From equation (i) and (ii), we get,

Hence, the specific charge at the electron is determined if the values of E, B, and V are known.

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