An alcohol hydrometer is a tool used to measure the specific gravity of liquids, particularly in the brewing and winemaking industries. It helps determine the alcohol content in a beverage by floating in the liquid and providing a reading based on how deep it sinks. The scale on the hydrometer indicates the density of the liquid compared to water. To use an alcohol hydrometer, you first fill a test jar with the liquid and gently lower the hydrometer into it. The point where the surface of the liquid touches the scale indicates the specific gravity, which can then be converted to find the alcohol by volume (ABV) percentage.