An electrodynamometer is an instrument used for measuring the electric power. The basic principle was laid out in an 1848 paper by Wilhelm Weber (1804-1891): when the same current passes through two concentric coils placed at right angles to each other, the resulting torque depends on the square of the current. 

   The electrical inventor and entrepreneur, Werner von Siemens (1816-1892), used this principle in his electrodynamometer, first described in 1880. In order to measure the power dissipated in an electrical load, it is necessary to measure the current through the load and the potential drop across it. In the Siemens instrument, the stationary coil is made of relatively few turns of heavy wire and is connected in series with the circuit. The rotating coil consists of many turns of fine wire, and is connected across the load with a multiplier resistance in series with it to measure the potential drop.  The currents through the two coils are I and a current proportional to V, and the product of the two currents is proportional to the power dissipated in the load. 

   The instrument above, at Wesleyan University, was built by R. H. Hopkins of the class of 1894 during the summer of 1897.

   The two instruments at the left and center, below were certainly imported by Queen of Philadelphia,
                  Wesleyan University                                Miami University                       St. Patrick's College, Maynooth, Ireland

   The electrodynamometer at the left is at Depauw University, and is the best preserved I have seen. This is probably due to the fact that a glass cylinder surrounds the mechanism, and protects it from dirt and mechanical damage. 

   This apparatus was imported by Queen & Co. of Philadelphia, and cost ca. $100 about 1880.

   The electrodynamometer at the right is one of the few pieces of apparatus that I have seen manufactuered by Elmer Willyoung of Philadelphia. It can be dated by noting that he sold his business to Morris E. Leeds of the same city in 1898; in 1903 Leeds joined forces with Edwin Northrup to form the long-lived firm of Leeds & Northup.

   The device is in the collection of the Virginia Military Institute in Lexington, Virginia.

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