A vinometer is a piece apparatus that consist of an open, graduated glass tube with a small filling reservoir.
The vinometer's reservoir or funnel is filled with a small amount of wine being tested until some wine exits out at the other end. It is then turned around and placed on a flat surface, filling resevoir side down, and allowed to self drain.
The alcohol concentration level is read of the vinometer's scale, on the capillary, at the top of the liquid being measured, in %v/v alcohol.
The level of the liquid is determined by the modifying affect that alcohol has on the interfacial tension between water and glass and the opposing surface tension of water. The greater the alcohol concentration the less marked the liquids capillary action and the lower it will sit in the tube of the vinometer.
The vinometer is calculated on the basis of pure alcohol and water solutions, whereas wort is not a pure water/alcohol solution and the accuracy of a vinometer can not be relied upon.
Sugar interferes with the interfacial effects that a vinometer's action relies on and hence the method can only be applied to fully fermented wort. Similarly other components in wort can also interfere with accurate measurements.
As a rough approximation of the wort alcohol content, the use of a vinometer with an internal triangular cross section has been reported to give improved accuracy.
Basic rules you have to keep in mind