The voltmeter is used to find the potential difference,it has high and finite resistance. While potentiometer has high and infinite resistance. More differences between potentiometer and voltmeter are given in the comparison table below.
Potentiometer Vs Voltmeter
|Its resistance is high and finite||Its resistance is high and infinite|
|It draws some current from emf source||It does not draw any electric current from the source of emf|
| Potential difference measured by it is less|
then the actual potential difference
|Potential difference measured by it is equal to|
actual potential difference
|It has low sensitivity||It has high sensitivity|
|It is a versatile instrument||It measures only emf or potential difference|
|It is based on the deflection method||It is based on the zero deflection method|
The potentiometer is a device mainly used to measure the emf of a given cell and to compare emfs of cells. It is also used to measure the internal resistance of a given cell.
Circuit diagram of potentiometer
Potentiometer consists of a long resistive wire AB of length L (about 6m to 10
m long) made up of mangnine or constantan. A battery of known voltage e and internal resistance r called supplier battery or driver cell. Connection of these two forms a primary circuit.
One terminal of another cell (whose emf E is to be measured) is connected at one end of the main
circuit and the other terminal at any point on the resistive wire through a galvanometer G. This forms the secondary circuit.
Other details are as follows
J = Jockey
K = Key
R = Resistance of potentiometer wire,
r = Specific resistance of potentiometer wire.
Rh = Variable resistance which controls the current through the wire AB
(3) Points to be remember
(i) The specific resistance (r) of potentiometer wire must be high but its temperature coefficient of
resistance (a) must be low.
(ii) All higher potential points (terminals) of primary and secondary circuits must be connected
together at point A and all lower potential points must be connected to point B or jockey.
(iii) The value of the known potential difference must be greater than the value of the unknown potential
difference to be measured.
(iv) The potential gradient must remain constant. For this, the current in the primary circuit must
remain constant and the jockey must not be slided in contact with the wire.
(v) The diameter of the potentiometer wire must be uniform everywhere.
See also : Joules law
Sensitivity of potentiometer : A potentiometer is said to be
more sensitive if it measures a small potential difference more
(i) The sensitivity of the potentiometer is assessed by its potential
gradient. The sensitivity is inversely proportional to the potential gradient.
(ii) In order to increase the sensitivity of potentiometer
(a) The resistance in the primary circuits will have to be decreased.
(b) The length of the potentiometer wire will have to be increased so that the length may be measured more
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