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
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