The magnetic field
dB due to this element is to be determined at a point P which is at
a distance r from it Let q be the angle between dl and the
displacement vector r According to Biot-Savart’s law, the
magnitude of the magnetic field dB is proportional to the current
I, the element length |dl|, and inversely proportional to the square
of the distance r Its direction* is perpendicular to the plane
containing dl and r