The field inside a solenoid with n turns per metre is given by
Notice that this predicts that the field is constant anywhere inside the
solenoid. Ideally this applies to an infinitely long solenoid. In practice this means a
solenoid where the length is large compared to the radius.
The distinction between a "coil" and a "solenoid" is rather vague i.e.
if you increase the radius of a solenoid, at what point should it be considered as a
circular coil? By making measurements with different combinations of coils you can
investigate this.
Helmholtz coils
This is an arrangement of two circular coils that is commonly used to produce a reasonably
uniform magnetic field. If the separation of the coils is equal to the radius of the coils then the
magnetic field at the centre of the arrangement is given by 