An element dl with current I produces a magnetic in duction dB= 0,dl×r1p0,sin6dl 4丌 4丌 where 0 is the angle between dl and rI, and 1 is the unit vector in the azimuthal direction Now using sin Adl= cos adl=rda and p=r cos a, sin odl rda da cos ada 2 Thus integration yields the magnetic induction B=4r//2 cos ada 4丌-m2p d12丌 See Fig 8-4