Now the way we do that is we essentially will have known values of R1, R2, and R3 and in particular what you will find is these values if we can change them, in known ways, we find the point at which the voltage between these 2 nodes labeled plus and minus 0 are equivalently we find the point  if you were to put a current meter, a galvanometer here where we measure it such that there is to current flow between these points in other words there's no voltage drop. 