J.J. Thomson's experiment to find the specific charge
In 1897 J.J. Thomson performed an experiment to determine the specific value of the charge(e/m) of an electron. The specific charge is the charge per unit mass.
PRINCIPLE:
J.J. Thomson used the principle of cross field to determine the value of specific charge of the electron. when a beam of electrons is subjected to a uniform electric and magnetic fields acting simultaneously and perpendicularly to each other in such a way that the deflection produced in one field is directly cancelled by the other. Hence the electrons passes undeviated .
In such case if 'E' is the electric field and the 'B' is the magnetic field applied on the electron moving with velocity 'v' then v=(E/B)
EXPERIMENTAL SETUP
Fig: Experimental setup for J.J. Thomson's experiment to find the specific charge
It consists of a highly evacuated. Discharge tube with a fluorescent screen. 'F' is filament being heated by a small battery. The filament heats the cathode which emits electrons.The electrons are then accelerated between the cathode and anode which are connected to a high voltage power supply of the order of 10 k v. The beam of electrons is then subjected to across field arrangement as shown in the field. The screen is coated with phosphorus so that when an electron strikes the screen a luminous point is seen.
WORKING AND OBSERVATION:
If both electric and magnetic fields are switched off the electron beam coming from the discharge tube strikes the screen at the centre O and the luminous spot is seen at the centre O. when a magnetic field alone is applied The beam is deflected downward and luminous spot is seen at O''. similarly when an electric field alone is applied the beam is deviated upward and spot is seen at the point O'. If both electric and magnetic field are applied at the same time and their values are so adjusted that their ratio (E/B) is equal to velocity of the electron the beam passes undeviated and spot is again seen at O.
If 'e' and 'm' be the charge and mass of the electron and 'V' be the potential difference between cathode and anode, then Kinetic energy (1/2mv2) gained by the electon is given by the electrostic potential (e V).
By substituting the values of E,V and B J.J. Thomson calculated the specific charge of the electron and found that specific charge of a electron has the value of 1.79 X 1011c/kg.
