X - Unknown Resistance,
R = Resistance box,
G = Galvanometer
AC = Uniform resistance wire,
Rh = Rheostat
D = Null point, E = Cell, K = Plug key
The key is closed to pass current by choosing suitable resistance R in resistance box. The jockey is tapped on the wire till a point D is located such that the galvanometer deflection is zero. The bridge is then said to be balanced.
From Wheatstone Bridge Principle,
As lx, R are known values, so the unknown resistance X can be calculated.
