Abstract: An electromagnet for producing a transverse field that is spatially periodic along an axis. Two coaxial interjacent helical poles are positioned on the axis. Electric current paths and magnetically conducting members connect to induce a magnetic flux of opposite polarity in the respective helical poles. A return path of magnetic material connects to all the conducting members thereby to complete the magnetic circuit. The resulting magnetic circuit produces a transverse field diametrically across the air gap formed by the two helical poles and this field is spatially periodic along the axis.