What instrument is used to measure specific gravity?

The instrument specifically used to measure specific gravity is a hydrometer. A hydrometer works by floating in a liquid; the level to which it sinks indicates the liquid's density compared to water. This measurement allows for the calculation of the specific gravity — which is a ratio of the density of the liquid to the density of water.

When using a hydrometer, the principles of buoyancy come into play. It floats at different levels depending on the density of the liquid it's placed in. When measuring a liquid with a known volume, the hydrometer can provide a direct reading of its specific gravity on its calibrated scale, which is specific to the measurement of liquids.

In contrast, the other instruments listed have different functions. A barometer measures atmospheric pressure, which is unrelated to the measurement of specific gravity. A manometer measures the pressure of gases, typically in terms of fluid height differences, and is not designed for comparing the densities of liquids. Lastly, a pycnometer also measures the density of liquids, but it does so by accounting for the volume of a known weight of liquid, leading to a more complex calculation to determine specific gravity rather than providing that measurement directly like a hydrometer does.