Like several other models made by the Jesuit mathematician Joseph F. MacDonnell of Fairfield University, this model shows a surface swept out by a moving straight line. Such a surface is called a ruled surface. It is represented by the cylindrical coordinates (r, t, z) by the equation r = a + z tan (pt/q). The model has a clear plastic cylindrical exterior, with pink plastic threads inside it that represent the surface. Here the ratio p/q = 1/3 and the fraction 1/3 is marked on the outside of the cylinder.
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