We start with a given circle.
|1. Place the right-angle corner of any object at any point on the circle. Any point will do.|
|2. Make a mark where the two sides of the right-angle 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 right-angle 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 right-angle 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.|