To calibrate a slide-wire potentiometer using a Weston standard cell, determine the uniform potential gradient along the slide wire, compare the electromotive forces of dry and primary cells at null balance without drawing current, and verify that cell EMF is strictly proportional to balancing length. Explore a virtual apparatus, collect readings and analyse a graph.
Ready for a virtual observation.
Adjust the apparatus, then take a reading.
Readings are generated by an ideal model. The diagram is schematic and is not to scale.
Calculations assume a perfectly uniform wire cross-section and resistivity, an unvarying driver accumulator current throughout all runs, negligible thermoelectric EMFs at metal junctions, and negligible contact resistance at the jockey slider. Real slide wires exhibit minor local diameter variations (~0.1%) caused by manufacturing tolerances or mechanical wear from slider friction, as well as parasitic thermal Seebeck EMFs at the copper-constantan binding posts if touched by warm fingers.
Provides a 1.000-meter uniform constantan or manganin alloy resistance wire (R_wire = 10.00 Ω, length L₀ = 100.0 cm) stretched above a parallel metric scale graduated in millimeters, mounted between massive copper terminal blocks of negligible contact resistance.
Supplies a steady, non-polarizing direct driving potential (ℰ₀ = 2.000 V) in series with a precision multi-turn wirewound rheostat to establish and maintain a perfectly constant potential gradient k = 2.000 V/m (0.02000 V/cm) along the slide wire.
Serves as the primary electromotive force calibration reference standard (ℰ_s = 1.0186 V at 20 °C) with an exceptionally low temperature coefficient of EMF (−0.00004 V/°C), balancing at ℓ_s = 50.93 cm.
Detects microampere-level imbalance currents (sensitivity ~10⁻⁷ A/div) between the wire contact potential and the cell EMF to identify the exact physical null balance position.
Protects the standard cell and delicate galvanometer suspension against destructive discharge currents during preliminary slider hunting, short-circuited during final delicate balance refinement.
Enables instantaneous two-way switching between the standard cell and unknown test cells, and provides smooth, repeatable electrical contact along the potentiometer wire without scraping or local mechanical deformation.
To calibrate a slide-wire potentiometer using a Weston standard cell, determine the uniform potential gradient along the slide wire, compare the electromotive forces of dry and primary cells at null balance without drawing current, and verify that cell EMF is strictly proportional to balancing length. Suggested independent settings: 0.4, 0.65, 0.9, 1.0186, 1.25, 1.5 V.
Connect the regulated 2.000 V driver accumulator in series with a master knife switch, an adjustable wirewound rheostat, and the 1-meter potentiometer slide wire AB. Ensure the positive terminal of the driver accumulator connects directly to binding post A (the zero of the metric scale). Close the switch and adjust the rheostat to set the potential drop across wire AB to slightly greater than the maximum cell EMF to be measured (setting the potential gradient to exactly k = 0.02000 V/cm).
Establishing a uniform, unvarying potential drop across the slide wire is the foundational requirement for all subsequent potentiometric null measurements.
Connect the positive terminal of the Weston cadmium standard cell (ℰ_s = 1.0186 V) to the common positive post A. Connect the negative terminal through the protective 10,000 Ω resistor and sensitive center-zero galvanometer to the movable knife-edge jockey slider F. Verify that the standard cell electromotive force directly opposes the potential drop along the slide wire from post A.
Connecting identical polarities to common terminal A ensures that the potentiometer wire potential and the test cell EMF oppose each other at the jockey contact, making null balance possible.
Lightly touch the jockey contact to the extreme left of the wire near post A (ℓ ≈ 1 cm) and tap the galvanometer key, observing the direction of needle deflection. Next, touch the jockey to the extreme right near post B (ℓ ≈ 99 cm) and tap the key. Verify that the two deflections occur in opposite directions. If opposite deflections are not obtained, reverse the cell connections or reduce the driver rheostat resistance.
Opposite galvanometer deflections at the wire extremes prove that the null balance point lies safely within the physical span of the slide wire and that polarities are correctly opposing.
Use the method of successive approximations (bisection search) to locate the approximate null point with the 10,000 Ω protective resistor in series. Once the needle deflection is less than 1 scale division, depress the bypass key to short-circuit the protective resistor, and gently adjust the jockey position until the galvanometer rests at perfect zero. Record the standard cell balance length ℓ_s = 50.93 cm and compute the potentiometer potential gradient k = ℰ_s / ℓ_s = 1.0186 / 50.93 = 0.02000 V/cm (2.000 V/m).
Calibrating against the certified Weston standard cell converts the linear geometric distance along the wire into an absolute thermodynamic potential scale.
Switch the secondary branch to the unknown test cells using the selector switch, ensuring that the driver circuit rheostat remains completely untouched. For each of the six cell EMF levels (spanning 0.400 V to 1.500 V, including standard cell, Daniell cell, and dry cells), determine the precise balancing length ℓ_x at galvanometer null. Record the balancing length in centimeters and convert to meters (L_x = ℓ_x / 100.0).
Keeping the primary driver current strictly constant throughout all runs preserves the calibrated potential gradient k across all cell comparisons.
Plot the balancing length L (in meters) on the horizontal axis and the cell electromotive force ℰ (in Volts) on the vertical axis. Compute ordinary least-squares linear regression to determine the potential gradient k (the slope m in V/m) and the vertical intercept b. Calculate the cell EMF ratios ℰ_x / ℰ_s and compare against the corresponding length ratios ℓ_x / ℓ_s to verify Poggendorff’s law of the potentiometer.
Linear regression over multiple cell EMF stations confirms the spatial uniformity of the wire and filters out visual meniscus and contact resistance uncertainties.
Points are your observations. The fit uses SI x and y values before display scaling.
| # | Test cell electromotive force ℰ (V) | Null balance length ℓ (cm) | Balancing length L (m) | Electromotive force ℰ (V) | Remove |
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