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:

`B=munI`

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.