A solenoid assembly comprising: (a) a core tube extending along a longitudinal axis, the core tube having a first ferromagnetic end section with a first confronting end longitudinally spaced from a second ferromagnetic end section with a second confronting end, a first non-ferromagnetic section adjacent the first confronting end and a second non-ferromagnetic section adjacent the second confronting end, a first ferromagnetic spacer (122, 1208, 1210) longitudinally intermediate the first non-ferromagnetic section and the second non-ferromagnetic section; (b) a first magnet and a second magnet located outside the core tube, the first magnet spaced along the longitudinal axis from the second magnet, wherein a second ferromagnetic spacer is longitudinally intermediate the first magnet and the second magnet, wherein the first non-ferromagnetic section is radially inward of the second magnet and the second non-ferromagnetic section is radially inward of the first magnet; and wherein the first ferromagnetic spacer (122, 1208, 1210) is radially inward of the second ferromagnetic spacer 128, 1202, 1204); and (c) an excitation coil disposed radially outward of the first magnet and the second magnet; characterised in that the second ferromagnetic spacer comprises a gap extending from a radial outer surface of the second ferromagnetic spacer to a radial inner surface of the second ferromagnetic spacer forming second ferromagnetic spacers (1202, 1204) each spaced apart from one another along the longitudinal axis by the gap, wherein the gap is spaced apart longitudinally from the first magnet and the second magnet; wherein a spring is in contact with the second ferromagnetic spacers (1202, 1204) configured to maintain the gap. (a) a core tube extending along a longitudinal axis, the core tube having a first ferromagnetic end section with a first confronting end longitudinally spaced from a second ferromagnetic end section with a second confronting end, a first non-ferromagnetic section adjacent the first confronting end and a second non-ferromagnetic section adjacent the second confronting end, a first ferromagnetic spacer (122, 1208, 1210) longitudinally intermediate the first non-ferromagnetic section and the second non-ferromagnetic section; (b) a first magnet and a second magnet located outside the core tube, the first magnet spaced along the longitudinal axis from the second magnet, wherein a second ferromagnetic spacer is longitudinally intermediate the first magnet and the second magnet, wherein the first non-ferromagnetic section is radially inward of the second magnet and the second non-ferromagnetic section is radially inward of the first magnet; and wherein the first ferromagnetic spacer (122, 1208, 1210) is radially inward of the second ferromagnetic spacer 128, 1202, 1204); and wherein a second ferromagnetic spacer is longitudinally intermediate the first magnet and the second magnet, wherein the first non-ferromagnetic section is radially inward of the second magnet and the second non-ferromagnetic section is radially inward of the first magnet; and wherein the first ferromagnetic spacer (122, 1208, 1210) is radially inward of the second ferromagnetic spacer 128, 1202, 1204); and (c) an excitation coil disposed radially outward of the first magnet and the second magnet; characterised in that the second ferromagnetic spacer comprises a gap extending from a radial outer surface of the second ferromagnetic spacer to a radial inner surface of the second ferromagnetic spacer forming second ferromagnetic spacers (1202, 1204) each spaced apart from one another along the longitudinal axis by the gap, wherein the gap is spaced apart longitudinally from the first magnet and the second magnet; wherein a spring is in contact with the second ferromagnetic spacers (1202, 1204) configured to maintain the gap. characterised in that the second ferromagnetic spacer comprises a gap extending from a radial outer surface of the second ferromagnetic spacer to a radial inner surface of the second ferromagnetic spacer forming second ferromagnetic spacers (1202, 1204) each spaced apart from one another along the longitudinal axis by the gap, wherein the gap is spaced apart longitudinally from the first magnet and the second magnet; wherein a spring is in contact with the second ferromagnetic spacers (1202, 1204) configured to maintain the gap.