Demonstration of Biot-Savart law. A wire carrying electric current generates a magnetic field around it. For a long straight wire the magnetic field lines form circles around the wire, as it follows from the Biot-Savart law. The magnetic field lines are visualized here by placing small compasses around a wire. When the current is not flowing, the needles point to the Earth's magnetic North (as shown in this photograph). When the current is flowing, the needles follow a circle around the wire. Photo 1 of 2.