A modern compass is a fixed compass: the distance between the two legs can be fixed so that it is possible to copy a line segment or a circle from one position to another (Fig. 1.1a).Euclid used a collapsing compass where a fixed distance cannot be maintained (Fig. 1.1b). Teachers often use a collapsing compass consisting of a marker tied to a string that is used to construct a circle on a whiteboard. It is impossible to maintain a fixed length when the compass is removed from the whiteboard.
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Ben-Ari, M. (2022). The Collapsing Compass. In: Mathematical Surprises. Springer, Cham. https://doi.org/10.1007/978-3-031-13566-8_1
Publisher Name: Springer, Cham
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