Cross-section of an infinite cylinder is shown. A current I uniformly distributed over its cross-section is flowing along its length. Now a cylindrical cavity of radius R/2,whose axis is parallel to the axis of the cylinder, is formed inside it. Then

In a cylindrical cavity like this, magnetic field is uniform and perpendicular to the line joining the centres of the cylinder and directed in the same direction.Magnitude of field at the centre of the cylinder as well as at the center of the cavity is μ0I3πR.  i.e., same both in magnitude and direction.