It contained Weston ammeters and voltmeters and Westinghouse circuit breakers. Measuring instruments such as ammeters and voltmeters need to be checked periodically against known standards. On heavy trains close monitoring of the ammeters was necessary to avoid motor damage. There is also a range of devices referred to as integrating ammeters. Zero-center ammeters are also commonly placed in series with a battery. Multirange ammeters that use this technique are more accurate than those using a make-before-break switch. Moving iron ammeters are commonly used to measure current in industrial frequency AC circuits. Early ammeters were laboratory instruments which relied on the Earth's magnetic field for operation. Everyday current meters for use in the field are called ammeters.