64(a)]
 This last equation relating the four resistors is called the balance
condition for the galvanometer to give zero or null deflection The Wheatstone bridge and its balance condition provide a practical
method for determination of an unknown resistance Let us suppose we
have an unknown resistance, which we insert in the fourth arm; R4 is
thus not known Keeping known resistances R1 and R2 in the first and
second arm of the bridge, we go on varying R3 till the galvanometer shows
a null deflection