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 The bridge then is balanced, and from the balance
condition the value of the unknown resistance R4 is given by,
2
4
3
1
R
R
=R R
[3