Question

What must be ideal toroid?

Answer 1

An ideal toroid consists of a long conducting wire wound tightly around a torus, a doughnut-shaped ring, made of a nonconducting material.

In an ideal toroid carrying a steady current, the magnetic field in the interior of the toroid is tangential to any circle concentric with the axis of the toroid and has the same value on this circle (the dashed line in Fig.). Also, the magnitude of the magnetic induction external to the toroid is negligible.