Final answer:
The tool used for measuring the specific gravity of oil is a hydrometer, which operates based on the principle of buoyancy.
Step-by-step explanation:
Specific gravity is the ratio between the mass of a volume of a substance, like oil, and the mass of an equivalent volume of water. The correct tool for measuring specific gravity traditionally is a hydrometer. This instrument partially submerges in the liquid sample, and the specific gravity is determined by the reading on the scale at the air-liquid interface. Since the specific gravity is a ratio of densities, it is a unitless number. The hydrometer works on the principle of buoyancy and is designed with a sealed hollow tube, typically made of glass, with a wider bottom portion and a weighted ballast for stability. A narrow stem with marked graduations allows for the reading of specific gravity based on how much of the hydrometer is submerged in the liquid.