Toroid
A doughnut-shaped ring wrapped with insulated wire that confines a uniform magnetic field completely within its circular core.
Governing formula
B = (μ₀ N I) / (2π r)
SI unit
T (Tesla)
In depth
Because the magnetic flux path forms a closed torus loop, the magnetic field outside an ideal toroid is identically zero, minimizing electromagnetic interference (EMI).
Examples in the real world
Toroidal power transformers and common-mode chokes in high-fidelity audio equipment and sensitive electronic instruments.