The volt (symbolized V) is the Standard International (SI) unit of electric potential or electromotive force. A potential of one volt appears across a resistance of one ohm when a current of one ampere flows through that resistance. Reduced to SI base units, 1 V = 1 kg times m² times s^-3 times A^-1 (kilogram meter squared per second cubed per ampere). Voltage can be expressed as an average value over a given time interval, as an instantaneous value at a specific moment in time, or as an effective or root-mean-square (rms) value. Average and instantaneous voltages are assigned a polarity either negative (-) or positive (+) with respect to a zero, or ground, reference potential. The rms voltage is a dimensionless quantity, always represented by a non-negative real number. For a steady source of direct-current (DC) electric potential, such as that from a zinc-carbon or alkaline electrochemical cell, the average and instantaneous voltages are both approximately +1.5 V if the negative terminal is considered the common ground; the rms voltage is 1.5 V. For standard utility alternating current (AC), the average voltage is zero (the polarity constantly reverses); the instantaneous voltage ranges between approximately -165 V and +165 V; the rms voltage is nominally 117 V. Voltages are sometimes expressed in units representing power-of-10 multiples or fractions of one volt. A kilovolt (symbolized kV) is equal to one thousand volts (1 kV = 10³ V). A megavolt (symbolized MV) is equal to one million volts (1 MV = 10⁶ V). A millivolt (symbolized mV) is equal to one-thousandth of a volt (1 mV = 10^-3 V). A microvolt (symbolized µV) is equal to one-millionth of a volt (1 µV = 10^-6 V). Also See: Ohm's Law, prefix multipliers, Standard International (SI) system of units, and voltage