 thumb|upright|hydrometer from practical physics thumb|upright|schematic drawing of a hydrometer. the lower the density of the fluid, the deeper the weighted float b will sink; the depth can be read off the scale a. a hydrometer is an instrument used to measure the specific gravity (or relative density) of liquids; that is, the ratio of the density of the liquid to the density of water. a hydrometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. the liquid to be tested is poured into a tall container, often a graduated cylinder, and the hydrometer is gently lowered into the liquid until it floats freely. the point at which the surface of the liquid touches the stem of the hydrometer is noted. hydrometers usually contain a scale inside the stem, so that the specific gravity can be read directly. a variety of scales exist, and are used depending on the context. hydrometers may be calibrated for different uses, such as a lactometer for measuring the density (creaminess) of milk, a saccharometer for measuring the density of sugar in a liquid, or an alcoholometer for measuring higher levels of alcohol in spirits. ==principle== operation of the hydrometer is based on archimedes' principle that a solid suspended in a fluid will be buoyed up by a force equal to the weight of the fluid displaced by the submerged part of the suspended solid. thus, the lower the density of the substance, the farther the hydrometer will sink. (see also relative density and hydrometers.) ==history== an early description of a hydrometer appears in a letter from synesius of cyrene to the greek scholar hypatia of alexandria. in synesius' fifteenth letter, he requests hypatia to make a hydrometer for him. hypatia is given credit for inventing the hydrometer (or hydroscope) sometime in the late 4th century or early 5th century.female inventors - hypatia of alexandria http://www.inventions.org/culture/female/hypatia.html the instrument in question is a cylindrical tube, which has the shape of a flute and is about the same size. it has notches in a perpendicular line, by means of which we are able to test the weight of the waters. a cone forms a lid at one of the extremities, closely fitted to the tube. the cone and the tube have one base only. this is called the baryllium. whenever you place the tube in water, it remains erect. you can then count the notches at your ease, and in this way ascertain the weight of the water.taken from the two-volume set of letters, essays and hymns of synesius translated by a. fitzgerald, published by oxford university press in 1926 and 1930. available on-line at: http://www.webcitation.org/query?url=http://www.geocities.com/athens/acropolis/516
Brewing Laboratory_equipment Laboratory_glassware Measuring_instruments