Abstract: Various forms of hydraulic cylinders having axial bores formed in their piston rods, and probes projecting axially from the closed end of the cylinder body with two spaced coils thereon are disclosed. The coils are connected into external circuitry which responds to change in inductance of either coil whereby to generate a reference signal upon the occurrence of preselected changing conditions of inductance of either coil caused by movement of that coil into or out of mutual induction with a shorted turn arrangement formed by or carried by the piston. The coils may be the drive coils of a displacement transducer arranged to indicate either end of the stroke of the piston. Alternatively either a supplementary shorted turn or an additional drive coil may be provided to generate an intermediate stroke reference signal. The reference signals may be used in operation of a system for effecting automatic continuous reciprocation of the piston.