The Fahrenheit hydrometer is a device used to measure the density of a liquid. It was invented by Daniel Gabriel Fahrenheit (1686–1736), better known for his work in thermometry.
Operation
The Fahrenheit hydrometer is a constant-volume device that will float in water. In the figure shown here, the hydrometer is floating vertically in a cylinder containing a liquid. At the bottom of the hydrometer is a weighted bulb and at the top is a pan for small weights. To use the hydrometer, one first accurately determines its weight (W) while it is dry. Next, the device is placed in water, and a weight (w) sufficient to sink a marked point on the rod to the water-line is placed on the pan. At that point, the weight of water displaced by the instrument equals W + w. The hydrometer is then removed, wiped dry, and placed in the liquid whose density is to be determined. A weight (x) sufficient to sink the hydrometer to the same marked point is placed in the pan. The density (D) of the second liquid is then given by D = (W + x) / (W + w).[1]
The Fahrenheit hydrometer can be made of either glass or metal.
The Nicholson hydrometer is similar in design, but instead of a weighted bulb at the bottom there is a small container ("basket") into which a sample can be placed.
See also
Hydrometer
References
See page 170 of School Physics by Elroy McKendree Avery, 1895, American Book Company, New York, from which much of this article is taken.
Hellenica World - Scientific Library
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