cobalt magnet 36 and washer 38 to the end of rod 30 which is located within tube 24 spacer 34 magnet 36 and washer 38 must have sufficient clearance with the interior of tube 24 to prevent their coacting with the closed end of tube 24 in the nature of a dashpot as the slide valve piston 14 rod 30 and magnet 36 move as a unit a pair of flats 34a and b are formed in otherwise circular spacer 34 to permit the flow of fluid past spacer 34 which is the largest diameter member in tube 24 the tube 24 is located in a sealed unpressurized chamber defined by tube 40 and end plates 42 and 44 which are sealed to tube 40 by 0 rings 43 and 45 respectively to provide a water tight fitting mounting plate 48 is bolted to end plates 42 and 44 by a plurality of bolts 49 and serves to fixedly locate end plates 42 and 44 with respect to each other end plates 42 and 44 are located with respect to tube 24 by mounting rings 52 and 56 which are secured to end plates 42 and 44 respectively by screws not illustrated mounting rings 52 and 56 are secured to tube 24 by set screws not illustrated when the structure located in tube 40 is properly located with respect to tube 24 a pair of bead chain sprockets 60 and 62 are spacedly mounted on mounting plate 48 bead chain 64 is strung over sprockets 60 and 62 so that they move together as a unit tube 24 is partially covered by a tubular shim.