In the neutral point topic , what is the orientation of the magnet?
The orientation of the magnet here with respect to earth's magnetic field is the magnet is perpendicular to the earth's magnetic field.
> Why do we prefer a potentiometer over voltmeter to measure the emf of cell?
When we use voltmeter in an electrical circuit it draws current from the circuit so it measures only the potential difference across the circuit but not the emf of cell. But if we use potentiometer, at null point no current is drawn from the cell. Also, the potential measured by the voltmeter is always less than emf.