Milliohm meters are capable of measuring very small resistances. Devices which use a four-wire Kelvin probe system pass a test current through the probe so that the voltage is sensed by the center pin. This test method eliminates contact resistance and minimizes the possibility of errors caused by the resistance of the leads. By definition, resistance or impedance is a measure of the degree to which a material or device opposes the flow of current. This value is equal to the voltage drop across the element divided by the current through the element. As their name suggests, milliohm meters measure resistance in milliohms, a unit of measure which equals 1/1000 of an ohm.