A toroid has a major radius and a minor radius and is tightly wound with turns of wire on a hollow cardboard torus. The figure shows half of this toroid, allowing us to see its cross-section. If , the magnetic field in the region enclosed by the wire is essentially the same as the magnetic field of a solenoid that has been bent into a large circle of radius . Modeling the field as the uniform field of a long solenoid, show that the inductance of such a toroid is approximately
