In this experiment we determine the specific charge value e/m by Thomson method. Cathode ray tube is the main component of this experiment. As you know cathode ray tube have three components first is electron gun second is deflector plates for horizontal and vertical deflection and third part is a fluorescent screen.
The electron gun comprises a filament. Which on heating provide thermal energy to the cathode. As a result electrons emits from the cathode surface. Which further moves towards the anode, the potential difference between the cathode and anode decide the kinetic energy. The deflecting plates are connected with the positive terminal horizontally and vertically.
So providing the potential to these plates one can deflect the beam in any direction. In Thomson method there are two process to determine the charge to mass value of the electron. In one case we use the two uniform circular coils while in second one the two bar magnets. The two bar magnets are used in this experiment. So how to originate the formula a complete theory is explained.