Potentiometer: working principle

 



 


1) Potentiometer is a system of long wire, with uniform cross-sectional area.



2) A source of potential V is connected in the series with a variable resistor R.



3) To compare the emf of two cells, they are connected parallelly as shown in the circuit at the same point A.



4) A jockey and a galvanometer is also connected as like the circuit.



5) When 1 & 2 key is closed and the jockey is slided over the wire.

Suppose at J1, the galvanometer shows no deflection, then


 e1  -   f l1  =  0


 e1  =   f l1       ............    3.54



Where f is the potential drop per unit length of the wire.

AJ1 distance is l1  



6) When 1 & 2 key are opened,

Then 3 & 2 key are closed



7) As like previous work the jockey is slided over the wire and found at J2,  the galvanometer shows no deflection.




8) if AJ2 distance is l2  

we found that


 e2  -   f l2  =  0


 e2  =   f l2       ............    3.60




9) Now