Which tool is commonly used to measure the specific gravity of a battery?

The hydrometer is the correct tool used to measure the specific gravity of a battery electrolyte. Specific gravity indicates the concentration of the sulfuric acid in the electrolyte solution, which is crucial for determining the state of charge and overall health of a lead-acid battery.

When using a hydrometer, the user draws a sample of electrolyte solution from the battery cell into the tool, which has a float that rises or falls based on the density of the liquid. This density relates directly to the specific gravity. By reading the scale on the hydrometer, one can determine if the battery is sufficiently charged, needs charging, or is potentially failing.

Other tools listed, like the multimeter, ampermeter, and voltmeter, serve different purposes. A multimeter is used to measure voltage, current, and resistance; an ampermeter specifically measures current; and a voltmeter is focused solely on measuring voltage. While they can provide useful information about a battery's electrical performance, they do not measure the specific gravity directly, which is why they are not suitable for this particular task.