Du Plantier's Square Root Extractor
This Demonstration shows square root extraction based on the identity . For example, to find the square root of 9, move along the axis to and along the axis until the mark on the sloping line is on the axis. The intercept of the sloping line will be if measured in the same units as the axis and the sloping line. If we scale the axis in a ratio with the axis, then the intercept will be at .
 L. A. Graham, Ingenious Mathematical Problems and Methods, New York: Dover, 1959, pp. 74–75.