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Internal Resistance of Primary Cell via Potentiometer

Experiment video

What you will learn

Determine the internal resistance (r) of a primary Leclanché/Daniel cell using a high-precision 10-wire electrical potentiometer.

Materials needed

10-Wire Potentiometer board with uniform resistance wires
10-Wire Potentiometer board with uniform resistance wires
₹50
What is it?
Ten-meter constantan wire potentiometer mounted on polished wooden board with brass return strips.
Specifications
Total Wire Length
10.0 meters (10 × 100 cm segments)
Wire Composition
Manganin / Constantan (uniform diameter)
Meter Scales
Double scale 0 – 100 cm and 100 – 0 cm
Base Dimensions
115 cm × 25 cm teakwood base
Storage Accumulator (Lead-Acid / Regulated 4V DC supply)
Storage Accumulator (Lead-Acid / Regulated 4V DC supply)
₹95
What is it?
Leclanché primary cell with porous pot, carbon electrode, and zinc rod in ammonium chloride electrolyte.
Specifications
EMF
1.45 Volts
Depolarizer
Manganese dioxide / carbon granules in porous pot
Electrolyte
20% Ammonium chloride (NH4Cl) solution
Jar
Heavy glass square jar
Leclanché Cell / Dry Cell (primary test cell)
Leclanché Cell / Dry Cell (primary test cell)
₹140
What is it?
Heavy-duty 2V lead-acid accumulator or regulated low-ripple electronic battery eliminator.
Specifications
Voltage
2.0 V DC constant output
Capacity
10 Ah low-discharge rating
Voltage Stability
± 0.05% over 1 hour run
Terminals
Heavy insulated screw posts
Sensitive Galvanometer with high resistance protection
Sensitive Galvanometer with high resistance protection
₹185
What is it?
Standard laboratory apparatus used in Internal Resistance of Primary Cell via Potentiometer.
Specifications
Category
Physics Apparatus
Grade
CBSE / NEP 2020 Laboratory Standard
Standard Resistance Box (1 to 20 Ω)
Standard Resistance Box (1 to 20 Ω)
₹230
What is it?
Standard laboratory apparatus used in Internal Resistance of Primary Cell via Potentiometer.
Specifications
Category
Physics Apparatus
Grade
CBSE / NEP 2020 Laboratory Standard
High Resistance Box (10,000 Ω)
High Resistance Box (10,000 Ω)
₹275
What is it?
Standard laboratory apparatus used in Internal Resistance of Primary Cell via Potentiometer.
Specifications
Category
Physics Apparatus
Grade
CBSE / NEP 2020 Laboratory Standard
Jockey, 2 one-way plug keys, and connecting wires set
Jockey, 2 one-way plug keys, and connecting wires set
₹320
What is it?
Standard laboratory apparatus used in Internal Resistance of Primary Cell via Potentiometer.
Specifications
Category
Physics Apparatus
Grade
CBSE / NEP 2020 Laboratory Standard

Tick items as you collect them. Tap + to see the product, specifications and buy options.

Stay safe

Electromotive force (emf) of the auxiliary driver battery must be strictly greater than that of the test primary cell.
Positive terminals of both the driver battery and primary cell must connect to the same starting terminal (A).
Jockey must never be dragged along potentiometer wires; press lightly to preserve uniform wire diameter.
Ensure balance length l1 > l2 is always observed.

Steps

0 of 7 steps done

1Connect the auxiliary driver circuit: battery positive to terminal A of potentiometer, negative through rheostat and key K1 to terminal B.
Observation
Assemble primary potentiometer circuit with 2V accumulator, rheostat, and key across 10-meter wire.
Safety
Ensure battery polarity is strictly aligned: positive of accumulator and positive of test cell both connect to terminal A.
2Connect primary cell positive to terminal A, negative to galvanometer, high resistance, and jockey.
Observation
Test dual-end deflection: jockey at 0 cm and 1000 cm produces equal and opposite galvanometer swings.
Safety
If deflection is unidirectional, check whether driver battery voltage is lower than test cell EMF.
3Connect a shunt resistance box R in parallel across the primary cell via key K2.
Observation
With shunt key K2 open (open circuit), slide jockey to find null balance length l1 = 760.0 cm.
Safety
Tap jockey gently without scraping wire surface.
4Keep K2 open (cell on open circuit). Touch jockey along wires to find balancing length l1 where galvanometer deflection is zero (emf balance).
Observation
Plug shunt key K2 with R = 5.0 Ω in resistance box; null point shifts backward to l2 = 580.0 cm.
Safety
Close shunt key K2 only momentarily during balance check to avoid rapid cell polarization.
5Close key K2 with a resistance R = 5 Ω unplugged in the shunt box.
Observation
Repeat with R = 4, 3, 2 Ω; calculate r = R·(l1 - l2)/l2 for each load setting.
Safety
Unplug main circuit key between trials to conserve battery.
6Find the new balancing length l2 where null deflection is achieved (terminal voltage balance).
Observation
Consistent internal resistance r = 1.55 Ω verifies potentiometric null-method accuracy.
Safety
Store chemicals and zinc rods of primary cell properly after experiment.
7Compute internal resistance r = R × (l1 - l2) / l2 across 5 different shunt resistor values (2Ω, 4Ω, 6Ω, 8Ω, 10Ω).

Tap + on a step to see what you will observe and any safety tips.

Record your observations

Shunt Resistance R (Ω)	Open Circuit Balance l1 (cm)	Closed Circuit Balance l2 (cm)	(l1 - l2) (cm)	Internal Resistance r = R(l1-l2)/l2 (Ω)
2.0 Ω	760.0 cm	425.0 cm	335.0 cm	1.58 Ω
3.0 Ω	760.0 cm	498.0 cm	262.0 cm	1.58 Ω
4.0 Ω	760.0 cm	545.0 cm	215.0 cm	1.58 Ω
5.0 Ω	760.0 cm	580.0 cm	180.0 cm	1.55 Ω
6.0 Ω	760.0 cm	605.0 cm	155.0 cm	1.54 Ω

Tap a highlighted cell to write your own observation.

Result and conclusion

Internal resistance r = R(l1 - l2)/l2 of the primary Leclanché cell is measured accurately as 1.45 Ω ± 0.05 Ω without drawing active current during open-circuit null balance.

r = R · (l1 - l2) / l2 = 5.0 · (760.0 - 580.0) / 580.0 = 1.55 Ω

Observation	What it means
Zero Current Advantage	Potentiometer draws zero current at null balance, measuring true electromotive force (EMF) without voltmeter loading error.
Terminal Voltage Drop	Introducing shunt resistance R causes cell internal current, reducing terminal voltage V and shifting balance length from l1 to l2.
Constant Internal Resistance	Mean internal resistance of Leclanché/Daniel cell r = 1.56 ± 0.02 Ω across different external load currents.
Driving Cell Condition	Auxiliary accumulator EMF (> 2.0 V) strictly exceeds test cell EMF (1.45 V), ensuring balance points lie within the 10m wire length.

Reciprocal Shunt Resistance (1/R) vs Reciprocal Balance Length (1/l2)

Sample readings. Replace with your own results.

Experiment photos

Before the experimentDuring the experimentResult