Gradeulator Circular Slide Rule Invented by Frank J. Thomas

Frank J. Thomas (1924–1976), a construction engineer from Topeka, Kan., invented the "Gradeulator" in the early 1970s. This circular slide rule was used to convert survey rod readings to sea level elevations, to determine the quantities of cut and fill required for earthwork, and to establish elevations for site grading and pavements.

The instrument has a square white plastic base with rounded corners. The base is covered with white cardboard that is marked with a circle divided into 100 equal parts, with each part divided into tenths. Four paper discs, each backed with metal, rest on the base. Each disc is divided along the edge into 100 equal parts, 50 for "cut" and 50 for "fill," and has three rings of numbers for 100-, 50-, and 25-foot grids. The second-largest disc is turned upsidedown and has ten square notches along its edge. The smallest disc has paper marked with scales on both sides of the metal, with the "cut" sections outlined in red ink. The instrument is held together with a metal screw and wing nut.

A clear plastic pointer also pivots at the screw. A piece of clear plastic screwed to the right corner of the base holds the rim of the three largest discs. The inside of the smallest disc is marked: GRADEULATOR (/) INSTRUCTIONS COPYRIGHT © 1973 by FRANK J. THOMAS. The base is marked: GRADEULATOR; PATENT PENDING Frank J. Thomas serial no. 5-73; LEGEND (/) BASE = Rod Reading (/) DISC #1 = SEA LEVEL, 0–100 ft. (/) DISC #2 (Notched) = SEA LEVEL, hundreds (/) DISC #3 = SEA LEVEL, thousands (/) DISC #4 = DEPTH cut or fill, and QUANTITY per grid.

According to donor Rita Thomas Dukes, Thomas handmade and sold these instruments from his garage. The name "Gradeulator" was trademarked from November 11, 1975, to April 6, 1982 (serial no. 73013213, registration no. 1024537). In 1973, Thomas applied for a patent, which was issued in 1976.

Reference: Frank J. Thomas, "Rotary Slide Rule for Topographic Calculations" (U.S. Patent 3,937,930 issued February 10, 1976).

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