26 DIRECT CURRENT PROCESS CONTROL INSTRUMENTS-NAMELY, UNIVERSAL CONTROLLERS, CONDITION ALARMS, VARIABLE CONDITION INDICATORS, REMOTE SETPOINT AND AUTO/MANUAL TRANSFER STATIONS, SETPOINT STATIONS, SQUARE ROOT EXTRACTORS, VARIABLE CONDITION RECORDERS, HIGHEST OR LOWEST SIGNAL SELECTORS, INPUT TRANSMITTERS, OUTPUT TRANSDUCERS, POWER SUPPLIES, AND CALIBRATION AND TEST SETS THEREFOR