Abstract: A magnetic detector for detecting a change of state in a pipeline, primarily a passage of a marking pig, is disclosed. The actuator has a detector housing and an actuator housing. A piston extends from the detector housing into the pipeline, and is moveable by passage of a pig. A primary permanent magnet coupled to the piston moves in response to the passage of a pig. The actuator housing is adjacent the detector housing and a secondary hammer magnet is located in the actuator housing adjacent the location of the primary magnet in the detector housing. As the primary magnet moves in response to the passage of a pig, the magnetic field produced by the primary magnet moves, interacting with the hammer magnet, which also moves. As the hammer magnet moves, its supporting linkage enters a field generated by a third booster magnet. The booster magnet attracts the hammer magnet and associated linkage toward it. A hammer is connected to the hammer magnet support linkage.