Variable reluctance electromagnetic sensor with pole piece-magnet assembly locking mechanism and method of assembly

A variable reluctance electromagnetic sensor comprising:

• tubular casing having a cavity;
• reference surface mark means on the outside of said tubular casing near one end thereof;
• an electrical coil wound around said casing, one end of said coil coincident with said reference surface mark means;
• an assembly comprising a permanent magnet and a pole piece inserted end to end in said cavity;
• said end of said pole piece opposite said permanent magnet being the free end, said free end being positioned a predetermined distance form said reference surface mark means;
• and
• locking means for wedging said assembly in said cavity, said locking means having a truncated conical surface progressively reducing the diameter of said cavity from a point intermediate the ends of said cavity towards an end of said cavity.