Figure A: When the power is cut off, there is no current in the circuit, and the small magnetic needle points to the north-south direction;

When Figure B is energized, current flows in the circuit, and the direction of the small magnetic needle deflects, indicating that the small magnetic needle is subjected to magnetic force, indicating that there is a magnetic field around the current.

Comparing Figure B and Figure C, there is current flowing, but the deflection direction of the small magnetic needle is different from that of the current, which means that the direction of the small magnetic needle is different, that is, the direction of the magnetic field is different, which means that the direction of the magnetic field is related to the direction of the current.

There are many devices that use the current magnetic effect, such as electromagnetic cranes and electromagnetic relays.

So the answer is: magnetic field; When the current direction changes, the magnetic field direction also changes (or the magnetic field direction is related to the current direction); Electromagnetic crane.