Introduction:
The direction of the magnetic field is determined by the movement of a North Pole of a compass needle. The compass needle aligns itself with the magnetic field lines and points towards the Earth's magnetic North Pole. This phenomenon can be explained by the interaction between the Earth's magnetic field and the magnetic properties of the compass needle.

Understanding the magnetic field:
- A magnetic field is an invisible force field generated by a magnet or a current-carrying wire.
- It is a vector quantity, meaning it has both magnitude and direction.
- Magnetic field lines are used to represent the direction and strength of the field.
- The magnetic field lines always form closed loops and are directed from the North Pole to the South Pole of a magnet.

Interaction with a compass needle:
- A compass needle is a small magnet with a North Pole and a South Pole.
- In the absence of any external magnetic field, the compass needle aligns itself parallel to the Earth's magnetic field.
- The Earth's magnetic field is approximately aligned with the geographic North Pole.
- Therefore, the North Pole of a compass needle points towards the geographic North Pole, which is actually the Earth's magnetic South Pole.

Movement of the compass needle:
- When a compass needle is brought near a magnet or a current-carrying wire, it experiences a force due to the interaction between the magnetic field of the magnet/wire and the magnetic field of the Earth.
- The compass needle aligns itself along the magnetic field lines of the external field.
- Since like poles repel each other, the North Pole of the compass needle moves towards the South Pole of the external magnetic field.
- Therefore, the direction in which the North Pole of the compass needle moves inside the magnetic field is considered as the direction of the magnetic field.

Conclusion:
The direction of the magnetic field is determined by the movement of the North Pole of a compass needle inside it. The compass needle aligns itself with the magnetic field lines, pointing towards the Earth's magnetic North Pole. This alignment is a result of the interaction between the Earth's magnetic field and the magnetic properties of the compass needle.