In thomson's experiments with electron beams, when the magnetic force fm balanced the force of the electric field fe, the velocity of the beam was given by the divided .

In Thomson's experiment, when the magnetic force balanced the electric force, the velocity of the beam was

where E is the magnitude of the electric field and B the magnitude of the magnetic field.

Let's see why. The force exerted by the electric field of intensity E on each electron is

where  is the electron charge.
The force exerted by the magnetic field of intensity B on each electron is

where v is the speed of the electrons. 

When the two forces are balanced, we have:

from which we find