where d ℓ ℓ is a vector along the path C whose magnitude is the length of the differential element of the wire in the direction of conventional current, ℓ ℓ is a point on path C , and r ′ = r − − ℓ ℓ is the full displacement vector from the wire element ( d ℓ ℓ ) at point ℓ ℓ to the point at which the field is being computed ( r ), and μ 0 is the magnetic constant. Alternatively: B ( r ) = μ μ 0 4 π π ∫ ∫ C I d ℓ ℓ × × r ^ ^ ′ | r ′ | 2 where r ^ ^ ′ is the unit vector of r ′ . The symbols in boldface denote vector quantities.