In a co-axial, straight cable, the central conductor and the outer conductor carry equal currents in opposite directions. The magnetic field is zero

Step-by-step solution

Since the current carried by the outer and inner are equal and in opposite direction, magnetic field produced by the cylinders at a point P(distance r from central axis) is ⇒ B → outer = μ o 4 π i d l → × r → r 3 ⇒ B → inner = - μ o 4 π i d l → × r → r 3 ⇒ B → = B → outer + B → inner = 0