An instrument for measuring the Density of a liquid. The density of the Acid in a car Battery, changes as the stored electrici charge in the battery, is used to power things. A Hydrometer is commonly used to measure the Acid Density in Lead/Acid car batteries. The result of the Hydrometer reading let's you know how much electric charge is left in the battery. It is most commonly used to find out whether or not, the battery is old and depleted, or still in good condition. Go to Wikipedia to find out what a Hydrometer looks like.
This is the instrument which is used to measure and record humidity