Two long parallel wires are at a distance 2d apart. They carry steady equal currents flowing out of the plane of the paper, as shown. The variation of the magnetic field B along the line $XX\text{'}$ is given by

If the current flows out of the paper, the magnetic field at points to the right of the wire will be upwards and to the left will be downward. Now, magnetic field at C is zero. The field in the region BX¢ will be upwards (+ve) because all points existing in this region are to the right of both the wires. Similarly, magnetic field in the region AX will be downwards (–ve). The field in the region AC will be upwards (+ve) because points are closer to A as compared to B. Similarly, magnetic field in region BC will be downward (–ve).