An interesting alternative, if the students are not allowed to ever measure the circles with a ruler, is to have them discuss among groups who has the best results and why. This activity is a nice analog to Rutherford's experiment where he found the size of gold nuclei (circles) by firing alpha particles (marbles) at a thin gold sheet (sheet of paper). Discuss with the students how Rutherford would have confirmed his results. He could not use a ruler to measure directly. He had to rely on the reproducibility of his results by other scientists. A student laboratory information sheet and a master page of circles are available.
This activity takes about one 55-minute period to complete.