What is one common characteristic of a refractometer and a hydrometer?

A refractometer and a hydrometer share the characteristic of measuring the specific gravity of liquids. Specific gravity is a dimensionless quantity, defined as the ratio of the density of a substance to the density of a reference substance, typically water for liquids.

A refractometer accomplishes this by measuring the refractive index of the liquid, which is influenced by the liquid's concentration and density. As the concentration of a solute in the liquid increases, so does the refractive index, allowing the refractometer to provide a reading that correlates with specific gravity.

On the other hand, a hydrometer directly measures specific gravity by floating in the liquid to a depth that corresponds to its density. The reading is taken from the scale on the hydrometer at the surface level of the liquid, which provides a direct indication of specific gravity.

In contrast, the other options do not accurately reflect the common functionalities of these two instruments. Both devices do not measure temperature, are not typically used for electrical testing, and while some refractometers may be digital, many are analog, and hydrometers are generally not electronic devices, making the conclusion that both are digital devices inaccurate. Therefore, the correct focus on the shared function of measuring specific gravity is the