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Principle: The potential difference between any
two points on the wire of a potentiometer is directly proportional to
the length of the wire between these two points.
potential difference between points A and P.
length of the wire between points A and P.
To measure e.m.f. of a cell using
potentiometer Circuit diagram
A cell whose e.m.f. is to be
measured is always connected in the secondary circuit.
l1 = l (AP) = balancing length for cell E1
L = Total length of the wire.
Comparsion of EMFs
l1= l (AP) = balancing length for E1
l3 = balancing length for (E1 + E2)
l2 = balancing length for E2
l4 = balancing length for (E1 - E2)
Difference between potentiometer and voltmeter:
Potentiometer measures e.m.f.
Voltmeter measures terminal potential
difference which is slightly less than e.m.f.
Potentiometer measures even small
Voltmeter cannot measure small
find the internal resistance of a cell using potentiometer
A cell whose
internal resistance is to be determined is connected in the secondary
Where r =
internal resistance of cell E1
resistance taken out from the resistance box.
when K2 is open.
when K1 is closed and resistance R is taken out from the