The four-wire resistance thermometer configuration increases the accuracy and reliability of the resistance being measured: the resistance error due to lead wire resistance is zero. In the diagram above a standard two-terminal RTD is used with another pair of wires to form an additional loop that cancels out the lead resistance. The above wheatstone bridge method uses a little more copper wire and is not a perfect solution. Below is a better configuration, four wire kelvin connection. It provides full cancellation of spurious effects; cable resistance of up to 15 Ω can be handled.
With the 4-wire configuration, the instrument will pass a constant current (I) through the outer leads, 1 and 4. The voltage drop is measured across the inner leads, 2 and 3. So from V = IR we learn the resistance of the element alone, with no effect from the lead wire resistance. This offers an advantage over 3-wire configurations only if dissimilar lead wires are used, and this is rarely the case.
