How is diameter measured?

Answer 1

Let us assume you have a circle drawn with the center identified. Then draw one straight line through the center. Measure the length of the line bound by the intercepts of the straight line with the circumference of the circle. The line segment is the diameter.

Another case would be that you have a circle drawn with no center marked. Draw one straight line through the circle. Use a compass to draw the perpendicular bisector of the line segment bound by the intercepts of the straight line with the circumference of the circle toward the inner circle (the center of a circle cannot lie outside the circle!). Repeat drawing another (different) straight line through the circle and finish with a perpendicular bisector. The two bisectors will intercept at the center of the circle. Then you can proceed the same way as described in the first paragraph above.

Hint to draw a perpendicular bisector of a line segment: take one end of the compass, pivot the point at one end of the line segment and mark an arc with the other end on both sides of the line. Move the compass and pivot one point at the other end of the line segment. Mark an arc with the other end on both sides of the line. If the procedure is done correctly, the two arcs, one from each end, should intercept on one side of the line. There is another intercept of the two arcs on the side of the line. Connect the two arc-intercepts with a straight line. Convince yourself that the line bisects the straight line at a right angle. This last line is the perpendicular bisector of the original line (The first and last lines form the perpendicular bisector of one another).

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