This is an instrument that is supposed to measure the alcoholic strength of a finished wine. It relies on capillary action - the tendency of a liquid to climb inside a narrow tube, or capillary, to a height that depends on various factors, including surface tension, viscosity and specific gravity. It works reasonably well with very dry wines, but with sweeter wines the dissolved sugar affects the result. Being a capillary, it is a difficult instrument to keep clean, and any deposits on the inside will also affect the capillary action. Don't bother buying a vinometer. The right way to measure alcohol content is with before and after hydrometer readings.