8. 1 The Magnetic Induction B See fig 8-1 a circuit carrying a current I generates the magnetic induction at the point Pas B X ri 4丌 where the integration is carried out over the circuit c and the constant uo is called the permeability of free space 10=4丌×10-7 tesla meter/ ampere Remark: the unit vector r1 points from the source to the point of observation i.e. from di to P. b has unit tesla