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AC voltage is the potential difference of an alternating-current signal. In this dial position, the meter is set to measure how much voltage the circuit provides that varies over time, rather than a steady DC value. For most digital meters, the reading shown is the RMS value—the effective voltage that would produce the same heating in a resistor as the AC signal does. To use it, place the red probe on the test point and the black probe on the return, with the circuit powered and within the meter’s voltage range. This is distinct from measuring DC voltage (steady value), resistance (which requires the circuit to be off), or current (measured in series).