Two-Wire Resistance Measurements
Resistance measurements are often made using the two-wire method. A test current is forced through the test leads and the resistance under test. The meter then measures the voltage across the resistance through the same set of test leads and calculates the resistance value accordingly.
The main measurement issue with the two-wire method, as applied to low resistance measurements, is that the total lead resistance is added to the measurement. Since the test current causes a small but significant voltage drop across the lead resistances, the voltage measured by the meter won’t be exactly the same as the voltage directly across the test resistance, and considerable error can result. Typical lead resistances exist in the range of 10mW to 1W, so it’s very difficult to obtain accurate two-wire resistance measurements when the resistance under test is lower than 100W. For example, using test leads with a 200mW combined resistance to perform a two-wire resistance measurement on a 1000mW resistor will result in a 20% measurement error in addition to that of the instrument.