Question

explain kelvins doble bridge

Answer 1

Kelvin's double bridge is a more accurate resistance measurement instrument. It is better than Wheatstone's bridge for measuring electrical resistances below 1 Ohm.

See the diagram.  

Unknown Resistance Rx is found by inserting it into the bridge structure as shown. Then it is calculated after balancing the bridge for no current through the detector D and then

     Rx = R2/R1  * Rs