What tool is used to check the specific gravity of a battery electrolyte?

A hydrometer is the correct tool for checking the specific gravity of a battery electrolyte because it is specifically designed to measure the density of liquids. The specific gravity indicates the concentration of sulfuric acid in the electrolyte, which directly correlates to the state of charge of the battery.

Using a hydrometer, the user can immerse the device into the electrolyte, where it floats at a level that corresponds to the specific gravity reading. This measurement is crucial for assessing the health of a lead-acid battery, as it can provide insight into whether the battery is fully charged, discharged, or in need of maintenance.

Other tools like a micrometer, thermometer, or barometer serve different purposes. A micrometer measures small distances or thicknesses, a thermometer measures temperature, and a barometer measures atmospheric pressure. None of these tools can provide the specific information needed about the electrochemical state of a battery, making the hydrometer the appropriate and specialized choice for this task.