1. The problem statement, all variables and given/known data Wheatstone bridge. 2. Relevant equations i Why the voltage across whetstone's bridge isn't always zero ? according to ohm's law . It must be zero (V=I*R, R=0) =>V=0 across the bridge . 3. The attempt at a solution I think I'm missing something . the following sentence isn't necessarily true : delta V=0 doesn't => current=0 . I could explain this according to newton's first law . because V=0=> E=0 => current could exist in a Continuation state and flow with no help from external force( electric field). and we actually deal with the resistance across the winston's bridge as ideal and equals to zero. and this means voltage across the bridge is always zero . By calibring the resistance . We are only trying to fix kerchiefs first rule in order to have current equal to zero across the bridge ? I'm sorry ,I might not be clear because my english isn't good . But,in brief my problem is: 1) Do we deal with the resistance across Wheatstone bridge as ideal=0 or not? 2) In general if delta V=0 . Does this necessarily mean that current=0 ? in other words could there be a case that current flow when dela V=0 (maybe when R also equals to zero...)?