ANSWER:
Function of Magnetic Field:
When an accelerated beam of isotopic positive ions is allowed to pass through a strong magnetic field, the field makes them move in a circular path. The ions are, thus, separated on the basis of their m/e value, and fall on the electrometer to develop the current.
(Ref. An Insight Into Objective Chemsitry-11, Ch-01, Q. No. 24)