An apparatus comprising: a ferromagnetic housing defining a housing cavity; an electrically-conductive coil disposed in the housing cavity and defining a coil cavity; a ferromagnetic core piece disposed in the coil cavity; a ferromagnetic pole piece comprising: a first face in contact with the core piece, and a projection extending from a second face of the pole piece opposite the first face; a flexible element defining an opening, wherein: the flexible element is a guide spring in the form of a disk spring; the projection is disposed within the opening, and the guide spring is disposed between a portion of the projection and the first face of the pole piece; and a ferromagnetic armature coupled to the guide spring, characterized in that, the guide spring is disposed between at least a portion of the armature and the first face of the pole piece, the guide spring is welded to a back face of the armature; the apparatus further comprises a valve spring retained by a poppet which is inserted into the armature and swaged in place, wherein the valve spring is a disk spring; and a body having an adjustable threaded connection with the housing configured such that threading the housing further into the body has the effect of increasing a preload on the guide and valve springs and vice versa so as to vary an initial opening current. a ferromagnetic housing defining a housing cavity; an electrically-conductive coil disposed in the housing cavity and defining a coil cavity; a ferromagnetic core piece disposed in the coil cavity; a ferromagnetic pole piece comprising: a first face in contact with the core piece, and a projection extending from a second face of the pole piece opposite the first face; a first face in contact with the core piece, and a projection extending from a second face of the pole piece opposite the first face; a flexible element defining an opening, wherein: the flexible element is a guide spring in the form of a disk spring; the projection is disposed within the opening, and the guide spring is disposed between a portion of the projection and the first face of the pole piece; and the flexible element is a guide spring in the form of a disk spring; the projection is disposed within the opening, and the guide spring is disposed between a portion of the projection and the first face of the pole piece; and a ferromagnetic armature coupled to the guide spring, characterized in that, the guide spring is disposed between at least a portion of the armature and the first face of the pole piece, the guide spring is welded to a back face of the armature; the apparatus further comprises a valve spring retained by a poppet which is inserted into the armature and swaged in place, wherein the valve spring is a disk spring; and the guide spring is disposed between at least a portion of the armature and the first face of the pole piece, the guide spring is welded to a back face of the armature; the apparatus further comprises a valve spring retained by a poppet which is inserted into the armature and swaged in place, wherein the valve spring is a disk spring; and a body having an adjustable threaded connection with the housing configured such that threading the housing further into the body has the effect of increasing a preload on the guide and valve springs and vice versa so as to vary an initial opening current.