In Thomson experiment the electron is deflected downwards when only magnetic fied is applied (This deflection need to be left or right side). Why this happens? . But there is a logic/reason for electron to be deflected when the electron passes through electric field .

motion of achrged particle in presence of magnetic field
If a positively charged particle moves with an initial uniform velocity in a direction perpendicular to a uniform magnetic field, B, it will move in a circle of radius R in a plane at perpendicular to the magnetic field, with a constant speed v. The radius R is given byR = mv/|q|B
where |q| is the absolute value of the charge on the particle of mass m.

Approved