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Straightedge
From Wikipedia, the free encyclopedia
- For the subculture, see Straight edge.
A straightedge is a tool similar to a ruler, but without markings.
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An idealized straightedge is used in compass-and-straightedge constructions in plane geometry. It may be used:
- Given two points, to draw the line connecting them.
- Given a point and a circle, to draw either tangent.
- Given two circles, to draw any of their common tangents.
It may not be marked or used together with the compass so as to transfer the length of one segment to another.
It is possible to do all compass and straightedge constructions without the straightedge. That is, it is possible, using only a compass, to find the intersection of two lines given two points on each, and to find the tangent points to circles. It is not, however, possible to do all constructions using only a straightedge. It is possible to do them with straightedge alone given one circle and its center.
Another way of doing this is described in the link at the end of the article. It involves simultaneously making three straightedges from appropriately sized bar stock of metal or wood. The bars are sequentially compared to each other in both directions and small amounts of material are removed from the stock to improve the fit. When all three bars match in both directions, the procedure has been successful in producing the three straightedges.
- Wayne R. Moore, Foundations of Mechanical Accuracy, Moore Special Tool Company, Bridgeport, CT (1970)
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