Patent Model for Combination Rule, Protractor, and Calculating Device by Nels Ockerlund

This combination instrument may be used to measure distances of up to 24 inches, to measure angles, to solve problems relating to ratios, to assist in finding the area of triangles when the lengths of three sides are known, and to add three-digit numbers. It is in the general shape of a four-fold rule, with a wooden core covered on both sides with German silver. A scale of inches, divided to 1/16" and numbered by ones from 1 to 12 twice, runs along the outer edges of the arms. Along the inner edges of both arms are scales of equal parts, with 30 parts to the inch.