A cylindrical cavity of diameter $$a$$ exists inside a long solid cylinder of diameter $$2a$$ as shown in figure. Both the cylinder and the cavity are taken to be infinitely long. The axis of the cavity is parallel to the axis of the cylinder and is at a distance $$\frac{a}{2}$$ from it. A uniform current of current density $$J\,\text{A m}^{-2}$$ flows through the cylinder along its length and not through the cavity. The magnitude of the magnetic field at a point $$P$$ on the surface of the cylinder lying farthest from the axis of the cavity, is