Dynamo Model
   The ideal dynamo shown [below] is arranged so that at every revolution of the crank the ratchet is lifted, allowing a spring to rotate the coil through 10°. The terminals of the coil are connected to separately insulated copper rings attached to the shaft. Copper strips or "brushes" bearing upon this ring are connected to a moving-coil galvanometer. As the coil moves through any ten-degree interval, an E.M.F. is induced which corresponds to the mean angular displacement of the coil from its original position. Since the total resistance through which this E.M.F. causes a current to flow remains the same through the experiment, the quantity of electricity passing through the galvanometer, and therefore the galvanometer deflection, is proportional to this E.M.F. If the observed deflections are plotted as ordinates and the corresponding values of the angular displacements as abscissas, the smooth curve drawn through these points will in general have practically a sine form.
   From Robert Andrews Millikan and John Mills, A Short University Course in Electricity, Sound and Light (Finn and Company, Boston, 1908) pg 134

   This example was made by Wm. Gaertner & Co. of Chicago and is in the Greenslade Collection. Almost identical models were made for many years by the Central Scientific Company of Chicago and the W.M. Welch Scientific Company of Chicago. The1928 Cenco catalogue listed it at $50.00.

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