Construction and Working of Wheatstone Bridge

Construction and Working of Wheatstone Bridge Points : Construction and Working of Wheatstone Bridge, Use of Wheatstone Bridge A very important device used in the measurement of medium resistances is the Wheatstone bridge. Following fig. shows the construction and working of Wheatstone bridge. The basic circuit of Wheatstone bridge has four resistive arms, together with a source of emf (a battery) and a null detector, usually a galvanometer “G” with a switch “s”. It may be a sensitive current meter. The current through the galvanometer depends on the potential difference between points “C” and “d”. The bridge is said to be balanced when there is no current through the galvanometer or when the potential difference across the galvanometer is zero. This occurs when the voltage from point “C” to point “a” equals the voltage from point “d” to point “a”; or, by referring to the other battery terminal, when the voltage from point “c” to point “b” equals the voltage from point “e” to point “b”. For balanced bridge (on closing the switch if galvanometer shows no deflection), the following conditions also exist. There are two parallel paths between points “a” and “b”, hence two currents in the circuit. “Ii” is the current in upper path through “P” and “Q” known ratio arms. “I₂” is the current in lower path through “R” (unknown resistance) and “S” (variable resistance).

When bridge is balance
I₁P = I₂R (i)
And I₁Q = I₂S (ii)
Dividing expression (i) by expression (ii) we have,

P/Q = R/S
or
R= P/Q.S

This is the well known expression for the balanced Wheatstone bridge. If three of resistances are known we can find the value of unknown resistance by this expression. Use The Wheatstone bridge is used for measurement of resistance from 1.0 to 1,00,000Ω, with a possible extension to this range at reduced sensitivity. The accuracy of the bridge almost solely depends on the quality of the resistors.