Cathode Ray Deflection Tube

The deflection tube by magnetic effect or cathode ray tube may be used to measure the specific charge (e/m) of the electron. This is a most useful quantity - the ratio of the charge to mass of the electron. For the electron the value of the specific charge is 1.76 x 1011 C kg-1

The Deflection tube
This tube uses crossed electric and magnetic fields and is very similar to that used by Thomson in 1897. The electrons are accelerated through a potential V by the electron gun and are then acted on by an electric and magnetic field placed at right angles.
This means that the electric and magnetic forces oppose each other. If the current in the coils and the voltage across the plate are adjusted so that the electric force (Ee) and the magnetic force (Bev) are equal