It is difficult to obtain highly accurate readings of current and voltage in small resistances. The difficulty exists both in analog as well as digital meters. Part of the reason is that their sensor wires and fine probe points add unknown amounts of resistance to the measurement, potentially more than that of the device under test. Additionally, sensor current must be carried over the probe lines that supply the current to be measured to the meter.
To resolve these difficulties, the great Civil War-era experimenter and theoretician, William Thompson, also known as Lord Kelvin, invented the Kelvin Bridge. The purpose was to measure low resistance using four-terminal sensing. In addition to the typical two-wire connection, another two-wire probe circuit known as a Kelvin connection is added. When four-wire Kelvin connections are used, current is supplied via two of the connections. They generate a voltage drop across the impedance to be measured using the other two connections.
Additionally, a pair of sense connections are made immediately adjacent to the target impedance, so that the voltage drop in the free leads is not included. Little current flows from the measuring instrument, so the voltage drop in the sense leads is close to zero.
The sense wires are on the inside paths, the force wires on the outside. If these connections are reversed, accuracy can be impacted, because more of the lead resistance is included. The force wires will be made to carry a large current when measuring small resistance, an occasion for error. This technique is used in low-voltage power supplies. Here it is known as remote sensing, since to measure the voltage delivered to the load independent of voltage drop in the supply wires enhances accuracy.
When a Kelvin connection is used, current is supplied via a pair of force connections (current leads). These generate a voltage drop across the impedance to be measured according to Ohm’s law V=IR. A pair of sense connections (voltage leads) are made immediately adjacent to the target impedance, so that they do not include the voltage drop in the force leads or contacts. Since almost no current flows to the measuring instrument, the voltage drop in the sense leads is negligible.
It is usual to arrange the sense wires as the inside pair, while the force wires are the outside pair. If the force and sense connections are exchanged, accuracy can be affected, because more of the lead resistance is included in the measurement. The force wires may have to carry a large current when measuring very small resistances, and must be of adequate gauge; the sense wires can be of a small gauge.
The technique is commonly used in low-voltage power supplies, where it is called remote sensing, to measure the voltage delivered to the load independent of the voltage drop in the supply wires.
It is common to provide 4-wire connections to current-sensing shunt resistors of low resistance operating at high current.