Which instrument is used to measure the specific gravity of a lead-acid battery's electrolyte?

The instrument that measures the specific gravity of a lead-acid battery's electrolyte is a hydrometer. This device works by floating a calibrated glass or plastic tube in the liquid electrolyte, which consists of a mixture of sulfuric acid and water. The hydrometer's scale allows the user to read the specific gravity directly, which is an important indicator of the state of charge of the battery.

A specific gravity reading gives insights into the concentration of sulfuric acid in the electrolyte; as a battery discharges, the specific gravity decreases because the sulfuric acid is consumed in the chemical reaction. Therefore, measuring the specific gravity with a hydrometer helps determine if a lead-acid battery is adequately charged or if it needs to be charged.

The other instruments listed serve different purposes. An ammeter measures electric current, a voltmeter measures voltage, and a multimeter can measure various electrical parameters, but none of these can determine the specific gravity of the electrolyte in a lead-acid battery.