Sometimes called an alcometer or alcohol meter.
A hydrometer specifically for testing alcohol levels in distilled spirits. It is only designed to work in solutions of ethanol and water. It is not meant to read the alcohol levels in beer or wine.
This hydrometer reads alcohol percentage (ABV) on a scale of 0-100%.
"Tralle's scale" measures proof on a scale of 0-200. 80 proof is equivalent to 40% ABV.