Nicholson's Hydrometer
   A hydrometer is a device for measuring the specific gravity of a liquid relative to water, which is assumed to have a specific gravity of 1.00000... Or, it can be thought of as measuring the density of the liquid in grams per cubic centimeter. 
   Nicholson's hydrometer differs from other  hydrometers in being totally submerged in use. The example at the left, from St. Mary's College in Notre Dame, Indiana, is pictured upside-down; in use all of the instrument except the small platform is immersed in the liquid. It is first placed in water and small weights loaded on the platform until the water level reaches a mark on the wire stem. The device is then placed in the unknown liquid, and loaded to the same line. The relative density of the unknown can then be obtained in terms of the mass of the hydrometer and the two sets of masses used to bring it to the reference line. 
   The apparatus at the right is in the Garland Collection at Vanderbilt University.
   The example of Nicholson's Hydrometer at the right is 
25 cm high, and is in the Greenslade Collection.

  William Nicholson (1753-1815) is today best remembered as the founder, in 1797, of the Journal of Natural Philosophy, Chemistry and the Arts, more commonly known as Nicholson's Journal. This he carried on until shortly before his death. His first piece of extended scientific writing was his 1781 book An Introduction to Natural Philosophy. In 1800 he utilized Volta's recently announced voltaic pile to decompose water into hydrogen and oxygen, thus establishing the science of electrochemistry.

   This apparatus, in the Millington/Barnard Collection at the University Museum at the University of Mississippi, is identified as Nicholson's hydrometer. It is proably the most expensive and versatile model listed in the 1853 Lerebours et Secretan catalogue, priced at 16 francs (about $3.20). 

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