Thompson Tube is used to study theeffect of electric and magnetic field on electron. A high voltage is appliedbetween anode and cathode. Electrons are therefore emitted from cathode andaccelerated towards anode. Cathode is maintained at ground potential. Soelectrons are emitted from cathode with a kinetic energy proportional to theanode voltage. These emitted electrons from cathode are then acceleratedtowards anode and pass horizontally through a slit behind the anode. Theseenergetic electrons then impinge upon fluorescent where they are observed. Aplate capacitor is used to displace the electrons vertically upward or downwardin fluorescent screen. Helmholtz's coils are used to produce a horizontalmagnetic field which moves the electrons in the vertical directions.