Hydrometer (third from left in photo)

This hydrometer’s (third from left in photo) glass bulb is anchored with lead shot at its lower end. The instrument contains a U.S. Internal Revenue tax stamp C4205, which features an engraving of George Washington, the same image found on the U.S. one dollar bill. The engraved stamp was inserted into the bulb during manufacture by an agent of the Commissioner of Internal Revenue as authoritative proof of the scale’s accuracy. The number C4205 indicates the stamp was delivered to the Internal Revenue Bureau from the Bureau of Engraving and Printing on January 23, 1905. An accurate scale was essential for determining the alcohol content of wine for tax purposes.
Hydrometers are the fundamental tool for measuring the alcohol potential of freshly crushed grape must (fermenting liquid), as well as the progress of a fermentation (the process whereby sugars are converted to alcohol via yeast). Made of glass, the bulb is weighted to allow it to float upright when suspended in liquid. A hydrometer will float to a different height on the measuring stick due to differences in the density of the liquid being measured. Because sugar increases the density of water, it is possible to know how much sugar is present when the density changes, i.e., when sugars turn into alcohol, during fermentation.
Grape juice at harvest contains about 25% sugar, with lesser amounts in cool climate grapes. Winemakers take frequent readings of the must (fermenting liquid) during fermentation. When the density stops changing and it is near zero, the fermentation is finished. There is always a trace of remaining sugar, in the range of 0.1 to 0.3%, but this amount has to be measured by another, more sensitive means. If the fermentation stops before the density is near zero, for instance at 3% sugar, this is considered a stuck fermentation, and a problem for the winemaker. The wine will taste sweet, which is generally not a desired outcome, so the fermentation will have to be restarted. This is generally done by adding yeast from another, active fermentation.
This hydrometer was donated by the Department of Viticulture and Enology at the University of California at Davis.
Physical Description
glass (overall material)
ID Number
catalog number
accession number
Credit Line
Gift of the UC Davis Department of Viticulture and Enology thru David E. Block
See more items in
Work and Industry: Food Technology
FOOD: Transforming the American Table 1950-2000
Food: Transforming the American Table, 1950-2000
Exhibition Location
National Museum of American History
Data Source
National Museum of American History


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