Single coil
Theory suggests that the field strength, B at the centre of a coil of radius r and n turns is given by
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Measure the field strength for coils of different radii and plot a suitable graph.
The field along the axis of a circular coil is given by
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where r is the radius of the coil and x the distance along the axis. Make measurements and plot a graph to test this relationship.
Solenoid
The field inside a solenoid with n turns per metre is given by
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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
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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 ./programme-assets/