Potentiometer- An ideal voltmeter that does not change the original potential difference, needs to have infinite resistance. But a voltmeter cannot be designed to have infinite resistance. The potentiometer is one such instrument that does not draw any current from the circuit and still measures the potential difference. so it behaves as an ideal voltmeter.
"A potentiometer is an instrument. This is used to measure the potential difference between two points of an electric circuit and emf of a cell."
Principle-
The principle of the potentiometer depends upon the potential gradient along the wire i.e. "When a constant current flows in a wire then the potential drops per unit length of the wire".
Construction- A potentiometer consists of a long wire $AB$ of uniform cross-section, usually, this wire is $4 m$ to $10 m$ long and it is made of the material having high resistivity and low-temperature coefficient such as manganin or constant. Usually, $1m$ long-separate pieces of wire are fixed on a wooden board. These pieces of wires are parallel to each other and joined in series by thick copper strips. A meter scale is used to measure the position of the jockey on the wire and it is fixed parallel to the wire ( As shown in the figure). The ends $A$ and $B$ are connected to a strong battery, a plug key $(K)$, and a rheostat $(Rh)$. The circuit is called a driving or auxiliary circuit that sends a constant current $i$ through the wire $AB$. Thus the potential gradually drops from $A$ to $B$. This potential drop is measured with the help of a jockey and voltmeter. A jockey is used to make point contact on the wire and it slides on the wire along the length of the wire.
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| Diagram of Potentiometer |
Working-
When a constant current flow through a wire of uniform cross-section area and composition, the potential drop across any length of the wire is directly proportional to that length.
If the voltmeter is connected between the end $A$ and Jockey $j$, it reads the potential difference $V$ across the length of the wire $AJ$. So according to ohm's law-
$V=iR$
$V=i \rho \frac{l}{A} \qquad(1)$
for a wire $\rho,i,A$ are constants. So
$\frac{i \: l}{A}=K (Constant)$
Hence equation $(1)$ can be written as
$V=Kl$
$V \propto l$
