All Century DC and AC rectifier type meters listed have the Simpson self-shielded movement. (Calibration is not affected by stray magnetic field or magnetic mounting.) AC meters have either iron-vane type or rectifier type movements and are calibrated at 60 Hz. Iron-vane type ammeters and millimeters have frequency range (for +/- 1% deviation from value of 60 Hz) of 25 to 800 Hz. Iron-vane voltmeters have range of 25 to 125 Hz. Calibration to 800 Hz available on special order. All rectifier types have 25 Hz to 10 kHz range. All AC ammeters over 10 Amps require a current transformer. Most DC ammeters 50 Amps or less have an internal shunt. All DC ammeters over 50 Amps require a DC shunt.