Hoopion

Meet Hoopion, a ring of wire who turns a circling current into a field that shoots straight through the middle.

Bend a wire into a circle and pass a current around it. Each little piece of wire makes its own magnetic field, and at the centre of the loop all of those contributions point the same way, straight through the hole. The loop behaves like a tiny flat bar magnet with a north face and a south face.

At the centre the field strength is B = μ₀I/(2R). A loop of radius 5 cm carrying 5 A gives B ≈ 6.3 × 10⁻⁵ T, which is about the same as the magnetic field of the Earth at the surface.

Use more current or a smaller loop and the field gets stronger. Stack many loops together and you have a coil, the basis of electromagnets.

Curl the fingers of your right hand along the current and your thumb points along the field through the loop.