hi, accod. to wheat stone bridge, null point means that the potential drop across x resistance = potential drop across legth of wire up to the null point. let length r La, Lb Lc resp. for the 3 null points measuring form left hand corner. then, x/La = R1/L-La,(as resistance r parallel) similarly x/Lb =R2/L-LB and so on. so u can know the value of unkwown. accuracy will be in B part as the % error in measuring the length will b minimum.