We place two or more compasses around a straight wire carrying an electric current (under, above, next to).
The needle of the compass: turns until it sits in an orthogonal direction with respect to the wire. The direction indicated by the needle is the opposite for the compasses placed beneath the wire, with respect to those placed above the wire.
If we reverse direction of the current in the wire the orientation of the needles is reversed in the compasses placed above and below the wire.
The magnetic effect of an electrical current is manifested on a perpendicular plane in the direction of the current. The magnetic field produced by a current extends above and below the cable. The direction of this is fixed when the direction of the current is fixed.
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