The coil space had a volume of 2.78 x 10-6 m3, and contained 200 turns of 0.4 mm diameter insulated copper wire, secured with a varnish layer on the outside. The coils were wired in antiparallel, so that the field in the plane of symmetry of the two coils was zero, but the field gradient was non zero. The coils were powered by a Quad™ audio frequency amplifier, fed from the internal reference of a lock-in amplifier. The coils were attached, and centred on the pole pieces with a 6.35 mm centred hole in the iron pole pieces. Proximity of the iron pole pieces has the effect of introducing a frequency dependence of the amplitude and phase of the gradient field. One way of overcoming this problem was to monitor the actual gradient field, with an additional set of coils, placed inside the coils used to generate the gradient [8, 14].