We start with a given circle. 

1. Place the rightangle corner of any object at any point on the circle. Any point will do.  
2. Make a mark where the two sides of the rightangle cross the circle.  
3. Draw a line between these two marks. Because of Thales Theorem, this is a diameter of the circle.  
4. Place the rightangle corner of the object at any other point on the circle. Any point will do, but for greatest accuracy, make it about a quarter the way round the circle from the first point.  
5. Make a mark where the two sides of the rightangle cross the circle.  
6. Connect these two points with a straight line. This is the second diameter.  
7. Done. The point where the two diameters intersect is the center of the circle. 
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