Hart's Inversor

Similar to Peaucellier linkage, Hart's linkage (Hart's inversor or Hart's cell) employs inversion to convert between circular and rectilinear motion. Compared to the Peaucellier linkage, Hart's device uses fewer rods.

The device consists of four rods AB, BC, CD, and AD, such that AB = CD and BC = AD and AD and BC intersect. O, P, Q on AB, AD, and BC satisfy

for 0 < m < 1. In ABD, (1) implies OP||BD. Similarly, in ABC, AQ||AC. Because of the symmetry, or since ABC = ADC), the quadrilateral ABDC is an isosceles trapezoid, so that BD||AC. It then follows from (1) that the three points O, P, Q are collinear and belong to a line parallel to both AC and BD.

The following property of the configuration will be proved later:

It indicates that P and Q are mutually inverse under an inversion with center O. This means that, if O is fixed and P traces a curve, Q will trace the inverse image of the curve. If an additional rod SP is so attached to the configuration that

and S is fixed, then P will trace a circle that passes through the center O of inversion. It follows that Q will then describe a segment of a straight line.

The applet below demonstrates this property. The points A, B, D, O, and S are draggable for the purpose of defining (or redefining) the attributes of the configuration. However, when P is dragged both O and S remain fixed.

Note that the dimensions of the rods impose certain limitations on the relative positions of the rods. When these are about to be violated while P is being dragged, the applet stops tracing the points. If this happens, return P into the arc already drawn and reconfigure from here.

However, by the Pythagorean theorem,

Hence from (4),

Further

Combining (5) and (6) we obtain

References

1. R. Courant and H. Robbins, What is Mathematics?, Oxford University Press, 1996
2. H. Rademacher and O. Toeplitz, The Enjoyment of Mathematics, Dover, 1990

Inversion

• Angle Preservation Property
• Apollonian Circles Theorem
• Archimedes' Twin Circles and a Brother
• Bisectal Circle
• Chain of Inscribed Circles
• Circle Inscribed in a Circular Segment
• Feuerbach's Theorem: a Proof
• Four Touching Circles
• Hart's Inversor
• Inversion in the Incircle
• Circle Inversion: Reflection in a Circle
• Circle Hart's Inversor
• Inversion with a Negative Power
• Peaucellier Linkage
• Polar Circle
• Radical Axis of Circles Inscribed in a Circular Segment
• Steiner's porism
• Three Tangents, Three Secants

Copyright © 1996-2008 Alexander Bogomolny