A Nicholson hydrometer is a type of hydrometer commonly used for measuring the specific gravity or density of liquids, typically in the context of determining the alcohol content in spirits or the sugar content in solutions. It consists of a hollow metal floatation body, an upper loading pan where additional weights can be added for calibration, and a weighted lower cone that helps the hydrometer to float upright in the liquid being tested.

The brass construction ensures durability and resistance to corrosion, making it suitable for use in various environments. The precision and craftsmanship in its finish are essential for accurate measurements, as any imperfections could affect the hydrometer's buoyancy and consequently its readings.