The instrument used to construct (or draw) circles is called a pair of compasses, although it is often known either as compasses or simply as a compass.
A pair of compasses usually consists of a joined pair of "legs", one of which has a sharp point, and the other of which holds a pencil. The sharp point is pushed lightly into the paper and acts as a pivot, while the other leg can be swung around it to draw an arc, or a complete circle.
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Use a compass. Not the kind that shows you where you are in reference to North but the kind from elementary school with a sharp needle point on one end and a pencil on the other. There is a angular ruler so that you can determine the radius and then you hold down the point and turn the pencil!