Abstract: An apparatus for comparing the distances of two balls from a third ball which all lie on a horizontal plane comprises a telescopic rod pivotally mounted to swing around the vertical axis of a stand which rests on the same plane as the balls. At the free end of the telescopic rod a vertical strip is attached which is brought to successively contact said two balls for comparing their distances from the third ball. The telescopic rod stand is provided with means for centering it vertically over the third ball. A dolly is inserted between the end of the telescopic rod and said vertical strip for supporting the rod, which dolly can be moved along a circle centered on the axis of said stand; said dolly being provided with a rack-and-pinion device for micrometrically approaching said strip to a ball.