For a better understanding is necessary to present the basic configuration of the sensing element. At the end of the sensor lead wires are welded and insulated, and the assembly is inserted in a metallic tube. Since the resistance value that allows the determination of temperature is only the sensor resistance and not the resistance of the connecting wires, there are three basic schemes for sensor connection that circumvent the inevitable fact of having to use lead wires to measure the sensor resistance.
2-Wire Connection: (second left picture)
The Wheatstone bridge has a resistance thermometer with two wires connection. In the equilibrium condition of the bridge, which occurs when the galvanometer "G" indicates zero, one gets the following relationship between the resistance:
Rsensor + RL1 + RL2 = R1.R3 / R2
If R3 = R2 then Rsensor + RL1 + RL2 = R1. Since R1 is known, it is determined the value of Rsensor + RL1 + RL2.
Clearly, the value of sensor resistance values are increased resistance of the connecting wires, which decreases the quality of the measurement with the resulting error in temperature measurement.
3-Wire Connection: (third left picture)
In this case the bridge is connected to a three wires resistance thermometer. Considering again that R2 = R3, the balance of the bridge will result from the condition:
Rsensor + RL2 = R1 + RL1
How is it possible, with good approximation, consider approximately equal to RL1 RL2 RL3 approximately equal to, the effect of the wire is much attenuated.
This is the type of connection used in resistance thermometers for industrial applications.
4-Wire Connection: (on the right picture)
The four wire system connection completely eliminates the resistance of the connecting wires, being used in situations that require very precise measurements. PRT elements are manufactured with four wires connection.
This configuration is intended for use in laboratories, reading instruments of reference sensors allowing the circulation of a constant current (usually 1mA) through the sensor element and the potential difference measuring are made on the sensor terminals thus eliminating the effect of the resistance of the leads in the final result.