Magnetic Effect of a Current
A current-carrying conductor produces a magnetic field around it.
A straight current-carrying wire produces a concentric circular magnetic field lines around it.
The magnetic field is stronger when closer to the wire. Thus, the field lines near the wire are closer together.
The direction of magnetic field is again determined by the Right Hand Grip Rule. However, now the thumb is the direction of current and curling fingers represents the direction of magnetic field.
Note that do not confuse with the Right Hand Grip Rule learnt in Magnetism.
Compass A is below the wire;
Compass B and C are besides the wire;
Compass D is above the wire.