|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 source of emf|
| Potential difference measured by it is less|
than 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|
Potentiometer is a device mainly used to measure emf of a given cell and to compare emf’s of cells. It is also used to measure 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 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 known potential difference must be greater than the value of 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 potentiometer wire must be uniform everywhere.
Sensitivity of potentiometer : A potentiometer is said to be
more sensitive, if it measures a small potential difference more
(i) The sensitivity of 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 primary circuit will have to be decreased.
(b) The length of potentiometer wire will have to be increased so that the length may be measured more