J.J. Thomson Method (Find Specific Charge of the Electrons)

J.J. Thomson Method:

Principle- This method is based on the facts that-

• Stream of negatively charged particles (electrons) constitute current.
• The stream of negatively charged particles is deflected by both electric field as well as magnetic field.

Construction- It consists of highly evacuated glass tube G. At one end of the tube, filament F made of Tungsten is fitted. The filament F is heated with a low tension battery (L.T.), and hence it emits electrons. The emitted electrons are accelerated by high voltage V applied between anode A and cathode C.

The accelerated electrons emerge through the hole in the anode in the form of a narrow beam. The electric field E can be applied perpendicular to the electron beam, with the help of metal plates D and E. The dotted circle represents the magnetic field. The magnetic field is applied perpendicular to both the electric field as well as to the electron beam. S is a screen coated with some fluorescent material ZnS. When the electron beam strikes the screen, a luminous spot is seen there.

Working and Theory-

Force acting on electron due to the electric field is given by-

Force acting on electron due to the magnetic field is given by-

Suppose at the start, no electric field or magnetic field is applied. Hence the electron beam will go straight and a bright spot is seen at M. Apply both electric field and magnetic field. Now, the electron beam will be deflected and a bright spot is seen at R. Now adjust the values of the electric field and magnetic field, so that bright spot is again seen at M. At this stage,

The K.E. gained by the electrons in going from cathode to the anode is given by-

Knowing the values of E, V and B, the value of e/m can be calculated. It is found that the value of e/m comes to be 1.77 x 10¹¹ c/kg. It should be noted that e/m is also called a specific charge of the electrons.