Computes the analytic magnetic field and/or magnetic field gradient due to a circular current flowing in a coil placed on the surface of the body. This is used to validate squid measurements by comparing them with those generated by a non-magnetic field. As the magnetic field is symmetric about the coil axes, by assuming that the coils are at the centre of the SQUID dewar we calculate the magnetic field at a distance specified by the radius between the dewar centre and the coil. The coil positions must therefore be entered as data points along with the squid sensor positions.