Kelvin's double bridge may be used for precision mesurement of four-terminal low resistances.
Four terminal resistors have two current leading terminals and two potential terminals across which the resistance equals the marked nomial value. This is because, the current must be enter and leave the resistor in a fashion that there is same or equivalent distribution of current density between the particular equipotent surfaces used to define the resistance. The additional points also eliminated any contact resistance at the current lead-in terminals.
- cc: Current terminals. (As shown in the above fig.)
- X : Unknown low resistance.
- S : Standard low resistance.
- A, a----B, b are variable medium resistances, inside the unit.
- B, b - are ganged resistor pair.
- Under balance condition