What is a toroid? Obtain the formula for the magnitude of the magnetic field due to a current-carrying toroid.

(N/A) toroid is a device consisting of a large number of turns of insulated wire wound on a hollow ring.
$A$ solenoid bent into the form of a closed ring is called a toroidal solenoid.
Let it carry a current $I$.
By Ampere's circuital law,we consider a circular Amperian loop of radius $r$ inside the toroid. The magnetic field $\vec{B}$ is tangential to the loop at every point.
According to Ampere's circuital law:
$\oint \vec{B} \cdot d\vec{l} = \mu_0 I_{enclosed}$
For a loop of radius $r$ inside the toroid,the total current enclosed is $N I$,where $N$ is the total number of turns.
$\oint B dl = B (2 \pi r) = \mu_0 N I$
Therefore,the magnitude of the magnetic field inside the toroid is:
$B = \frac{\mu_0 N I}{2 \pi r}$
If $n = \frac{N}{2 \pi r}$ is the number of turns per unit length,then $B = \mu_0 n I$.