The coefficient of viscosity is defined as "the tangential stress per unit area per unit of relative velocity between two surfaces separated by unit distance, the space between them being filled with the liquid in question" (from the 1940 Central Scientific Company catalogue). The most favorable experimental geometry is provided by placing the liquid between two concentric cylinders, one moving and one at rest.

   In the apparatus at the left, the outer cylinder is rotated at rates ranging from 50 to 150 rpm, and the resulting twist of the torsion wire suspending the inner cylinder is measured using light reflected from the small mirror at the bottom of the wire. Once the torsion constant of the wire is measured, the torque and hence the tangential stress can be calculated. This apparatus is listed as a piece of Welch apparatus in the 1935 catalogue of the Braun-Knecht-Heimann Co. of San Francisco and cost $19.00 without the drive motor. The device is in the collection of Kenyon College in Gambier, Ohio.

   The apparatus at the right is listed as a viscosimeter in the 1940 Central Scientific catalogue and cost $40.00 with the electric heater to change the temperature of the liquid. It still contains traces of oil from the last time it was used (well before 1964), and is missing the inner cylinder that is rotated by a string wound around a large pulley at the top of its shaft and then passing over the pulley on the base to a falling weight. The graph of the tangential velocity as a function of the (steadily-) falling mass is a straight line.
   Here is a third viscometer from the Kenyon College collection. It is like the one at the right above, but does not have the option of heating the fluid under test. 

   In this case the rotating cylinder is in place, so this device could be used once more. Note the release toggle on the upper right-hand side of the apparatus for letting the system start to spin. 

   The apparatus is unmarked.

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