Potentiometer
 
                               

A long piece of uniform wire (Few meters ) cut in several pieces placed side by side and joined with the help of metal strips and connected with battery , rheostat at  key is known as  POTENTIOMETER.

Let the current in the circuit is  I ( which can be  varied with the help of variable resistance ).

==> Now we can find potential difference between P&Q.

                       
 

Applications .....

(1) To compare e.m.f of two cells.

                                   
                
Here we can connect one cell with potentiometer at a time with the help of two way key.

Let say that we connect cell (1) [ E₁ ] with potentiometer .Actually initially we have connected only the positive terminal of cell with point P . Now we will search a point on potentiometer wire such that Galvanometer shows  zero deflection.

Q ...>

Now What does zero deflection of galvanometer means ????


Actually we tried to find two points on the potentiometer ( p & R) such that  ..>


V_p = potential of positive plate of cell [ E₁ ].


V_R = Potential of negative plate of cell [ E₁ ].

So there will be no current through the cell and galvanometer shows zero deflection .


In this case : 


V _PR = potential difference  between points  P & R .

                       
 
(2) To find the internal Resistance of a cell ...

                       
 

We will repeat the same thing , will find null point ( Zero deflection ) but this time we will mention about two circuits,( 1) Internal circuit ( Potentiometer ) and ( 2)  external circuit ( cell with resistance  in parallel).

Now find the null point when k₂ is open (no current in external circuit)

       

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