A slide-wire potentiometer measures the EMF of an unknown cell,
PHYSICS · Multi-part · Fermi University Physics · Scientific Practice & Experimental Methods · Uncertainty in measurement · Designing and running experiments
Measurement and Method, Ch Designing the measurement: noise, averaging, null and differential methods
Problem
A slide-wire potentiometer measures the EMF of an unknown cell, εx, by balancing it against a standard cell of certified EMF εs=1.0183V. A steady current from a driver battery is passed along a uniform resistance wire; a galvanometer connected to the cell under test shows whether any current flows into that cell. The standard cell balances (zero current) at a length ls=84.3cm along the wire; with the same driver current still flowing, the unknown cell balances at lx=91.7cm.
((a)) Explain why, at balance, the potential drop along the wire up to the balance point equals the cell's EMF exactly, and hence find εx.
((b)) A calibrated digital voltmeter of similar sensitivity to the galvanometer could instead be connected directly across each cell in turn and read on its scale. List which rows of the resulting uncertainty budget vanish when the potentiometer's null balance is used instead, and which remain.