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Optical measuring instrument (also refractive index meter) with which the concentration of liquids or solids can be determined by means of light refraction. A variant of such a device was developed by the German physicist Ernst Abbe (1840-1905), who was the owner of the Zeiss company. In viticulture, this makes it possible to determine the sugar content in the form of must weight and thus potential alcohol content already in the grapes in the vineyard or in the grape must. In a drop of grape must enclosed between two prisms, the light is diffracted according to the sugar content and the value is usually displayed together on a scale on the eyepiece in a unit of measurement that varies from wine-growing country to wine-growing country (for example Brix, Oechsle and KMW = Klosterneuburger Mostwaage). The more concentrated the sugar solution, the more the light is refracted.

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