A solenoid is a long helically wound wire coil which has a very small diameter as compared to its length and carries a uniform current. Each loop is coated with insulating material so that loops are insulated from each other.
The magnetic field inside (or in the center of) a solenoid is uniform. The field outside is weak and divergent. This is true for a long solenoid.
The magnetic field is given by the relation:
where, B is the magnetic field,
n is the number of turns per unit length (= total number of turns/solenoid length)
I is the current flowing through the solenoid
and `mu` is the magnetic constant.
The magnetic field is independent of the diameter and length of the solenoid.
This mathematical relationship is also known as Ampere's circuital law.