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 the magnetic field?

Fleming's left-hand rule is used to determine the direction of the magnetic field. The direction of electric current will be from the front wall to the back wall as it is opposite to the direction of electron flow. Since the direction of the force is towards the right side, by applying Fleming's left-hand rule, we can conclude that the magnetic field inside the chamber is in the downward direction.