Q6Electronic Measurement & Instrumentation

Construction: It consists of a uniform resistance wire (slide wire) stretched across a metric scale. A steady DC driver cell (usually a lead-acid battery) sends a constant working current through this wire via a rheostat. A highly accurate Standard Cell (like a Weston cell) is used for calibration. The unknown EMF source is connected through a sensitive galvanometer to a sliding contact (jockey) that moves along the wire.

Working Principle: 1. Standardization: The standard cell is connected. The jockey is placed at a specific length corresponding to its known EMF. The rheostat is adjusted until the galvanometer shows zero deflection, locking in the voltage drop per unit length of the wire. 2. Measurement: The unknown cell is then swapped in. The jockey is slid along the wire until a new null point is found. 3. Because the galvanometer draws zero current at balance, the unknown EMF is exactly equal to the voltage drop across that length of the slide wire: $V_{unknown}=k\times l$, where $k$ is the calibrated voltage drop per cm.