The specific gravity of electrolyte is measured by

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hydrometer

Explanation: The specific gravity of an electrolyte in a lead-acid battery is measured using a hydrometer. It serves as a direct indicator of the battery's state of charge, as the density of the sulfuric acid electrolyte changes during the charging and discharging process.

Working Principle: The hydrometer operates on Archimedes' Principle, which states that a floating body displaces a weight of liquid equal to its own weight. When the electrolyte is dense (fully charged), the hydrometer float rises higher in the tube. When the electrolyte is less dense (discharged), the float sinks deeper into the liquid.

Key Formulas: $SG = \frac{\rho_{electrolyte}}{\rho_{water}}$ — Definition of specific gravity; $State \ of \ Charge \propto \rho_{electrolyte}$ — Relationship between density and charge

Key Points: A fully charged lead-acid battery typically has a specific gravity between 1.250 and 1.280.; A discharged battery typically shows a specific gravity near 1.100 to 1.150.; Temperature correction is required if the electrolyte temperature deviates significantly from the standard (usually $27^\circ C$).; Sulfuric acid ($H_2SO_4$) concentration decreases during discharge as sulfate ions react with lead plates.

Advantages: Simple and inexpensive diagnostic tool.; Provides a reliable estimate of chemical state of charge.

Disadvantages: Requires manual sampling of the electrolyte.; Corrosive nature of acid poses safety handling risks.

Applications: Automotive lead-acid battery maintenance.; Stationary power systems and UPS installations.

Additional Info: Manometers are used to measure pressure, not density.. Hygrometers measure humidity, not electrolyte density.. Psychrometers are used in meteorology to determine dew point and humidity via wet/dry bulb temperature.

Answer: C is correct — The hydrometer is the standard instrument used to measure the specific gravity of the sulfuric acid electrolyte to determine the charge state of a lead-acid battery.

Exam Tip: Always remember that in lead-acid batteries, the specific gravity increases during charge and decreases during discharge, which is the inverse of the internal resistance change.

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