Well, no. If you have a circle, then have a line go through the circle, through any two points on the circle, in one end and out the other, i.e. NOT TANGENT, then you could find the center of that line with a protractor measured at the length of the line, put on one end then measure above and below the line and do the same on the other point. The two hemi-circles will intersect and use your handy straight edge to make a perpindicular bisector, this is the diameter. Then, find the center of this line with the same method of the protractor, measure above and below, find the perpindicular bisector and THAT is the center of the circle. I call it the freeman theorum.