The "SI" unit of electric resistance, reactance, and impedance. If a conductor connects two locations having different electric potentials, then a current flows through the conductor. The amount of the current depends on the potential difference and also on the extent to which the conductor resists the flow of current. For direct current circuits, this property of opposition to current flow is called the resistance. In alternating current circuits, the current flow is also affected by reactive components, capacitors or inductors, that react to the change in the current over time. This opposition is called reactance; impedance measures the combined effect of resistance and reactance. All three quantities are measured in ohms. One ohm is the resistance, reactance, or impedance that requires a potential difference of one volt per ampere of current. The unit honors the German physicist Georg Simon Ohm (1787-1854). The capital Greek letter omega is used as the symbol for the ohm, since "O" would be easily misinterpreted as a zero