to comprise an elongate core element or rod 40 of high magnetic permeability secured at one end by silver brazing to the bored upper end of a connecting rod 41 of non magnetic material such as brass rod 41 has a reduced end which is threaded and projects with radial clearance through a central hole in the shuttle wall 39 to enable axially secure fastening via lock nuts 42 with a measure of freedom for self alignment adaptability for rod 40 in stem 26 the core element 40 is slidably positionable within sleeve 26 and when shuttle 34 is seated at 37 33 the upper end of the core element 40 is at the elevation denoted n f in fig1 meaning the no flow condition when in this position core element 40 is totally lapped with a first 43 of two like coils 43 44 in this position core element 40 is also partially lapped with the second coil 44 as shown coils 43 44 are shown fitting in end to end abutting relation within a cup shaped coil shield 45 in telescoped overlap with the sleeve 26 and all fully contained within the cylindrical inner volume of the conduit boss 24 of the bonnet assembly 20 leads to respective coils 43 44 are shown extending through the open end of conduit boss 24 for wire nut or other wiring connection.