Final answer:
The instrument used to measure the relative density of the acid in a car battery is a hydrometer, which measures the specific gravity of the electrolyte to determine the battery's state of charge.
Step-by-step explanation:
The instrument used to measure the relative density of the acid in a car battery is called a hydrometer.
A hydrometer is an instrument designed to measure the specific density (or relative density) of liquids based on the concept of buoyancy. It consists of a sealed hollow glass tube with a weighted bottom and a narrow stemmed scale. When testing the density of battery acid, the hydrometer sinks at a level that correlates with the acid's density, with the specific gravity being readable directly from the scale marked on the hydrometer
This method is particularly important for monitoring the status of a lead-acid battery, as the specific gravity of the electrolyte changes when the battery charges or discharges. A higher specific gravity indicates a charged battery, while a lower specific gravity points to a discharged state. This measurement is done by observing the level at which the hydrometer floats within the electrolyte solution.