Kohlrausch Bridge
Friedrich Wilhelm Georg Kohlrausch (1840-1910) was a professor of physics at Göttingen, Zürich, Würtzburg, Strasbourg and Berlin during his career. He wrote a widely-used book on methods of experimental physics, and developed improved measurement techniques in electricity and magnetism. 
   The Catalogue and Price-List of Electrical Testing Apparatus, issued by James W. Queen & Co. of Philadelphia in 1887, shows the upper version of the Kohlrausch Universal Bridge (from the Denison University Collection). The catalogue description is:
   "Professor Kohlrausch's Universal Rheometer, or Bridge and Rheometer combined; with induction coil, for measuring the resistance of solid and fluid conductors, batteries, etc., by means of galvanometer or telephone ... $52.00." The telephone receiver was an extra $6.00. 

   The prefix rheo, introduced by Wheatstone in 1837, refers to resistance.

   The second piece of apparatus, from Washington and Lee University, can be found in the 1921 catalogue of E. Leybold's Nachfolger [Successors] of Cologne at a price of $38. The text reads:
   "Kohlrausch's universal bridge, allowing of reading the resistance of wires and electrolytes in ohms, with induction coil and 5 coils of 0.1, 1, 10, 100, 1000 ohms for comparison, available for resistances from 0.05 to 20000 ohms."

   The Kohlrausch bridge in the third picture is on display at the Garland Collection of Classical Physics Apparatus at Vanderbilt University. It was made by Hartmann & Braun of Frankfort am Main, and is clearly the same as the Queen apparatus from Denison University, showing that Queen was the importer. 

  The bottom picture shows a similar Hartmann & Braun Kohlrausch bridge in the collection of the University of Vermont in Burlington.


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