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Wheatstone bridge

12. Oct 2020
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Wheatstone bridge

1. Wheatstone Bridge:--The device uses for the measurement of minimum resistance with the help of comparison method is known as the Wheatstone bridge. Wheatstone bridge, also known as the resistance bridge. Used to calculate the unknown resistance by balancing two legs of the bridge circuit. It was invented by Samuel Hunter Christie in the year 1833, which was later popularized by Sir Charles Wheatstone in 1843. This bridge is very reliable as it gives accurate measurements. A Wheatstone bridge consists of two I / p and two o / p terminals includes of four resistors arranged in a diamond shape. Wheatstone Bridge:--
2. principle :-- The Wheatstone bridge works on the principle of null deflection, i.e. the ratio of their resistances are equal and no current flows through the circuit. The bridge is said to be in a balanced condition when no current flows through the galvanometer.
3. Construction of Wheatstone Bridge Ratio arms AB and BC :--A Wheatstone bridge circuit consists of four arms of which two arms consists of known resistances. One unknown resistance :-- One arms consist of an unknown resistance. Standard arm :-- One arms consist of a variable resistance. Galvanometer :-- The circuit also consists of a galvanometer .The current that flows through the galvanometer depends on the potential difference across it. An electromotive force source:-- The emf source is attached between point A and C while the galvanometer is connected between the points B and D. K K :---Keys used to complete circuit. K is attached between point A and C. K is connected be tween the points B and D.
4. Wheatstone Bridge and Its Working A Wheatstone bridge is widely used to measure the electrical resistance . When key K is pressed current I is provided by the cell. At point A current is divided into two parts one part I want passes through resistance P while other part I goes through resistance R .Now resistances P, Q , R and S are also adjusted that on pressing the key K there is no deflection in the Galvanometer G, it means there is no current in the diagonal BD .Thus same current will pass through resistance Q which is in resistance P and same current I will pass through S which is in resistance R.
5. Currents through the arms are assumed by applying Kirchhoff’s Junction Rule. Applying Kirchhoff’s Loop Rule for : Loop ABDA: -I1.P - Ig. G + (I - I1).R = 0 Loop BCDB: - (I1 - Ig).Q + (I - I1 + Ig).S + Ig. G = 0 When Ig = 0, the bridge is said to balanced. -I1.P + (I - I1).R = 0 - (I1 ).Q + (I - I1 ).S = 0 P Q = R S I1 I Ig I1 - Ig I - I1 I II I - I1 + Ig E A B C D P Q R S G
6. Note In balance condition potential at point B and D will be same so to find the equivalent resistance between A and C, the resistance connected between B and the can be neglected because no current flows through it. In balance condition if position of galvanometer and cell are interchanged then there will be no effect on the balancing state of the bridge. So the diagonal arms AC and BD of bridge are said to be conjugate arms. Greater is the deflection in Galvanometer with small deviation in balanced state of the bridge, greater is said to be sensitivity of the bridge. Bridge is maximum sensitive when resistance P Q R and S are the same order.
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