Why Use Sense Resistors in 4-Wire Kelvin Measurements?

[Tweedle_Dee]

In a 4-wire (kelvin) measuring system, why are there "sense" resistors? I understand that because there is very little current on the sense line there is no voltage drop. What purpose do they serve?

 

[waht]

Where are the sense resistors exactly? In a Kelvin 4-probe you run a constant current through a test resistance and then measure voltage drop across it.