3. Potentiometer Voltmeter
Potentiometer measure emf of cell
very accurately.
Voltmeter measure emf of cell
approximately.
Potentiometer does not draw any
current from known emf source while
measuring current.
Voltmeter draw current from known
emf source while measuring current.
While measuring emf, resistance of
potentiometer become infinite.
While measuring emf, resistance of
voltmeter becomes very high but
measurable.
In the potentiometer sensitivity is
high.
In the voltmeter sensitivity is low.
It is based on null deflection method. It is based on deflection method
Difference between Potentiometer and Voltmeter
4.
5. Potentiometer
•Potentiometer is an accurate circuit used to compare emf's of a cells , Potential
difference between two points of the electric wire .
•Potentiometer is based on the principle that potential drop across any portion of
the wire of uniform cross-sectional is proportional to the length of that portion of
the wire when a constant current flows through the wire
potentiometer which consists of a number of segments of wire of uniform area of
cross-section stretched on a wooden board between two copper strips .Meter
scale is fixed parallel to the length of the wire.
6. principle :--
The potentiometer works on the principle that when a constant current flows through a wire of uniform
cross sectional area, potential difference between its two points is directly proportional to the length of
the wire between the two points.
V α l or V = kl
Construction
Wooden Board :-- Aboard having copper strips wire and wooden scale .
Wire :-- A long wire of uniform cross-sectional area and of 10 m in length. The
material of wire should have a high resistivity and low temperature coefficient .
The wires are stretched parallel to each other on a wooden board. The wires are
joined in series by using thick copper strips.
Meter Scale :-- A wooden scale of 1 meter length fix on wooden board .
Battery :--A battery of known EMF V whose voltage is known as driver cell
voltage. A battery is connected across terminals A and B through a rheostat so
that a constant currents flows through the wire .
7. Key :-- Keys used to complete circuit .
Rheostat :-- A rheostat is an electrical instrument used to vary resistance, that usually consists
of a coil of wire with a terminal at one end and a sliding contact that moves along the coil to stop
the current.
Galvanometer :-- The galvanometer is the device used for detecting the presence of small
current and voltage or for measuring their magnitude. The galvanometer is mainly used in the
bridges and potentiometer where they indicate the null deflection or zero current.
Jockey :-- The jockey is used to slide on the bridge wire. It is a metal rod with one end as knife
edge.
Resistance Box :-- resistance box is that the variable resistances are available at one point. If
any circuit requires variable resistances, then there is no need of replacing the resistor.
Daniel Cell :--Daniel cell is a primary cell which cannot supply steady current for a long time. It consists of a
copper vessel containing a strong solution of copper sulphate. A zinc rod is dipped in dilute sulphuric acid
contained in a porous pot. The porous pot is placed inside the copper sulphate solution.
Leclanche Cell :-- A Leclanche cell consists of a carbon electrode packed in a porous pot containing
manganese dioxide and charcoal powder (Fig 2.22). The porous pot is immersed in a saturated solution of
ammonium chloride (electrolyte) contained in an outer glass vessel. A zinc rod is immersed in electrolytic
solution.
10. Suppose A and ρ are the area of cross-section and resistivity of the material of the wire the
resistance
ρ l
R = ----------------------------(i)
A
where l is the length of the wire
•If I is the current flowing through the wire then from Ohm's Law,
V = IR ------------------------------(ii)
Where V is the potential difference across the position of the wire of length l
Thus ,from ( I ) and (ii)
V = I R = I ( ρ l /A ) = k l
where K = ρ I /A
=> V is proportional to l when current I is constant
V
• K = is also known as potential gradient which is the fall of potential per unit length of
l wire
15. Sensitivity of a potentiometer depends on its potential gradient .
If the potential gradient of a potentiometer is small then the potentiometer is more
sensitive and hence more accurate .
Applications of Potentiometers
Potentiometer as a Voltage Divider
Audio Control
Television
Transducers
Precautions with Potentiometer:
1. emf of the cell connecting in primary circuit must be more than or equal to the
emf of the cell of secondary circuit otherwise zero deflection can not be obtained.
2. All the high potential points or positive terminals should be connected at A .
3. Balancing length should be calculated from A .
4. Area of cross-section of the wire should be uniform otherwise potential gradient
will not be constant .
5. Current should not be passing through potentiometer wire for a long time
otherwise this will heat up the wire and will changes its resistance and hence
potential gradient will also changed .ss
16. +
E1
E2
+
R.B
G
J1
l1
J2
l2
E
A
K
A
BRh
+
I
100
200
300
400
0
(A)Comparison of EMF's of two cells using potentiometer
• Positive terminals of two cells of emfs E1 and E2( whose
emf are to be compared ) are connected to the terminals
A and negative terminals are connected to jockey through
a two way key K2 and a galvanometer
• Now first key K1 is closed to establish a potential
difference between the terminals A and B then by closing
key K2 introduce cell of EMF E1 in the circuit and null point
junction J1 is determined with the help of jockey .
If the null point on wire is at length l1 from A then
E1 = K l1
Where K -> Potential gradient along the length of wire
17. • Similarly cell having emf E2 is introduced in the
circuit and again null point J2 is determined .
If length of this null point from
A is l2 then E2 = K l2
Therefore
E1 l1
=
E2 l2
This simple relation allows us to find the ratio of E1/E2
• if the EMF of one cell is known then the EMF of other
cell can be known easily.
18. (B) Determination of internal resistance of the cell
• Potentiometer can also be used to determine the internal resistance of a cell .
•For this a cell whose internal resistance is to be determined is connected to terminal A of the
potentiometer across a resistance box through a key K2
•First close the key K1 and obtain the null point .Let l1 be the length of this null point from terminal
A then
E = K l1
•When key K2 is closed ,the cell sends current through
resistance Box (R).If E2 is the terminal Potential difference
and null point is obtained at length l2(AJ2) then
• V = K l2
Thus
J
V
+
K
E
A
Rh
+
l cm
I
A
B
100
200
300
400
0
19. E l1
=
V l2
But E= I ( R + r ) and V = I R
This gives
E ( r + R )
=
V R
So ( r + R ) l1
=
R l2
giving
r = R ( l1 / l2-1 )
•Using above equation we can find internal resistance of any given cell .
0 l
V
20. Ammeter
•It used for the measurement of current.
•Its resistance value is less as compare to the
voltmeter.
•In-circuit it linked in series.
•This meter is more accurate than the voltmeter.
•Its range can not be varied.
Difference between ammeter and voltmeter
Voltmeter
•It used to find the value of voltmeter in any
circuit.
•Its resistance value is high than the ammeter.
•It linked in parallel combination in the circuit for
voltage measurement.
•Its accuracy is less than the ammeter.
•Its range can be varied.
