In Fig. 120 the circle represents a section of a shaft, the center of which it is desired to find. The corner of a square is placed on any point of the circumference. The points A and B are the intersections of the outer sides of the square with the circumference. Draw a line from A to B. Now shift the square a little, as represented by the dotted square, and with the corner on any other point mark the intersections A' and B', then connect A' and B', and the intersection of A B and A' B' will be the required center. It is necessarily the center, for it is the intersection of the two diameters.

Fig. 120 - Method of finding center of a circle.