4-wire resistance measurements are used when you need either very-high accuracy, or the measurement leads are very long. The way a normal 2-wire resistance measurement works is that a fixed current is passed through the resistance being measured and the meter reads the voltage drop across the unknown resistance, and Ohm's law allows the meter to calculate the resistance. The problem is that this technique read the total resistance (including the test leads and any connection resistance).
A 4-Wire resistance measurement uses seperate sets of leads for applying the current to the unknown resistance and for measuring the voltage drop across the resistance. This gives very accurate measurements because the sense inputs to the meter have very very high impedence,
resulting in very very small current flow through the sense leads. Thanks again to Ohm's law, very very small current flow means very very small voltage drop in the wires, resulting in very very small measurement error--regardless of how long the measurement leads are.
Or at least that's the theory.
Mike...
