Uses of Potentiometer
1) To compare Emf of a cell (Direct Method)
Construction:
Consider two cell of Emf E1 and E2 respectively whose emf is to be compared. Let E be the Emf of a battery Both cell is connected to Galvanometer via key and galvanometer is connected to Jockey on another side as shown by green colour which is tapped on Potentiometer wire. Let battery of Emf E is connected to Point A and B which is length of the potentiometer wire .
Working:
Case I : When Cell E and E1 are connected in system
Battery of Emf E and Cell of Emf E1 is introduced in the circuit. Cell of Emf E2 is not connected in first case. Now jockey is tapped on potentiometer wire to obtain a null point so that galvanometer shows zero deflection. Let point P be the point where Null point is obtained, such that Potential between point A and Q becomes equal to E1. Let point P obtained is at a distance of L1 distance from point A.
From Potential Gradient Equation,
VAP = σ L1
E1 = σ L1 --------------(1) E= V when null point is obtained.
Case II: When Cell E and E2 are connected in system
Battery of emf E and Cell of Emf E2 is introduced in the circuit. Cell of Emf E1 is not connected in this case. Now jockey is tapped on potentiometer wire to obtain a null point so that galvanometer shows zero deflection. Let point Q be the point where Null point is obtained, such that Potential between point A and Q becomes equal to E2. Let point Q obtained is at a distance of L2 distance from point A.
From Potential Gradient Equation,
VAQ = σ L2
E2 = σ L2 --------------(2) E= V when null point is obtained
From (1) and (2)
E1 / E2 = σ L1 / σ L1
E1 / E2 = L1 / L2
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