Question

how to compare emf of two primary cells by using potentiometer

Answer 1

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what is Potentiometer ?

It is an apparatus used for measuring the emf of a cell or potential difference between two points in an electric circuit accurately. When there is no potential difference between two points there no electric current will flow, this is the basic potentiometer principle.

To compare emf of two cells:The loop law is applied in comparison of emf of two cells.

In the  figure you see that,a current I flows through the wire which can be varied by a variable resistance (rheostat, R) in the circuit. Since the wire is uniform, the potential difference between A and any point at a distance l from A isε (l)= φ where φ is the potential drop per unit length.

When we connect 1 and 3 the cell with the emf E(1) gets connected with galvanometer G . Lets the null point be N(1) at length l(1) 

potential drop accros the length = φl(1) 

In the closed loop AN(1)G31A

                                             φl(1) + 0 - E(1) = 0 

Loop rule: The algebraic sum of changes in potential around any closed loop involving resistors and cells in the loop is zero.

Null point means there is no current in the AE(1)N(1) 

The emf of the cell will be equal to potential drop across the length 

Similarly, when we connect 1 and 2 cell with emf E(2)  comes in play.

                                        φl(2) + 0 - E(2) = 0 

⇒ E(1) / E(2) = l(1)  / l(2) 
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