A magnetic coil, especially for use as an r.f. transmit/receive coil in NMR apparatus, comprises a continuous closed conductive loop would round a former 1. A pair of external terminals 2 and 3 both lie in a plane 4 constituting a plane of symmetry and the coil is would so that there is mirror symmetry about this plane. There are two electrically parallel paths between terminals 2 and 3 each restricted to one side of the plane of symmetry 4 and the resultant magnetic field when a potential difference is applied between terminals 2 and 3 is such that equal magnetic fields are produced on both sides of the plane of symmetry 4.