63), we obtain,
1
4
2
3
I
R
I
R
=
Hence, we obtain the condition
2
4
1
3
R
R
R
=R
[3 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