Current flowing through a conductor produces a magnetic field. If the conductor is a long straight wire, then the field is distributed over a large region of space. If the wire is used to make a coil, the magnetic field is concentrated into a smaller space and is therefore stronger.

Theory suggests that the flux density, B_c, at the centre of a long coil, having N turns and of length . is given by

and, at a point on the axis near one end the flux density, B_e is

It can easily be demonstrated (using a "Hall detector") that if the length of the solenoid is at least 10 times its diameter, the flux density inside is uniform over most of its length.

The graph below show the variation of B along the axis of a long solenoid.

Points A and B represent the ends of the coil

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