Solenoid
A solenoid is a closely wound helix of insulated copper wire. Magnetic field at a point well inside a long solenoid is given by
B = μ o nl
where, n = number of turns per unit length and I = current flowing through the solenoid
Magnetic field at a point on one end of a long solenoid is given by B = μ o nl / 2
Toroid
A toroidal solenoid is an anchor ring around which is large number of turns of a copper wire are wrapped.
A toroid is an endless solenoid in the form of a ring.
Magnetic field inside the turns of toroid is given by B = μ o nl
Magnetic field inside a toroid is constant and is always tangential to the circular closed path.
Magnetic field at any point inside the empty space surrounded by the toroid and outside the toroid, is zero, because net current enclosed by these space is zero.
Magnetic Field Due to a Current Carrying Long Circular Cylinder
Outside the cylinder( r > R) B = μ / 2 π l / r Inside the cylinder when it is made of a thin metal sheet,