Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?

The Fleming’s Left hand rule can be used to determine the direction of the magnetic field. The magnetic field will be perpendicular to the current direction and the direction of deflection, i.e., either upward or downward. Because negatively charged electrons migrate from the back wall to the front wall, the current flows from the front wall to the back wall. The magnetic force is oriented rightward. Using Fleming’s left hand rule, it can be deduced that the magnetic field within the chamber is pointing downward.