Wright's Arithmeter Cylindrical Slide Rule

In the mid-19th century, the expansion and regulation of American insurance companies created a need for numerous computations and a demand for instruments to assist in this process. Elizur Wright (1804–1885), one of the first insurance commissioners of Massachusetts, invented this large cylindrical slide rule, patented it in 1869, and sold it to insurance companies for $500.00. It is the equivalent of a linear slide rule more than 60 feet long.

The instrument consists of two adjacent cylindrical brass drums, each covered with paper and mounted horizontally in a round brass frame, which is screwed to a round wooden base. Two indentations in the side of the base assist with lifting the instrument. A crossbar attached to the frame extends across the length of the drums. Two indicators slide across a groove in the bar. A brass handle with an ivory knob on the right side of the frame rotates the drums. An ivory button on the left side of the frame operates a brake. When the button is locked in a vertical position, the two drums turn together. When the button is horizontal, only the right drum turns.

The two cylinders are marked identically. Each drum has a spiral of 20 turns, divided logarithmically (perhaps by pencil), with printed numbers to the right of each division. The first digit of a number is read from the crossbar, and the remaining three are printed on the drum. The markings include every digit from 0 to 3,000; every even digit from 3,000 to 6,000; and every other even digit from 6,000 to 10,000. The arithmeter arrived in a badly scratched wooden case that has two metal handles and a keyhole (but no key).

A metal plaque screwed to the base is marked: No 6 (/) ELIZUR WRIGHT'S (/) ARITHMETER (/) PATENTED AUG. 17TH 1869. (/) N.E.M.L.INS. CO. The New England Mutual Life Insurance Company (now New England Financial), a company with a long connection to Wright and his family, donated this example, one of ten known surviving arithmeters. Wright's son, Walter C. Wright, was the firm's chief actuary from 1866 to 1900. Wright was also a well-known abolitionist. Although this example cannot be definitively credited to him, Joseph W. Fowle, a Boston machinist who invented a rotating rock drill, is known to have built some arithmeters for Wright.

References: Elizur Wright, "Calculator" (U.S. Patent 93,849 issued August 17, 1869); Peggy A. Kidwell, "Elizur Wright's Arithmeter. An Early American Spiral Slide Rule," Rittenhouse 4 (1989): 1–4; Lawrence B. Goodheart, Abolitionist, Actuary, Atheist: Elizur Wright and the Reform Impulse (Kent, Ohio: The Kent State University Press, 1990), 149, 168; Naom Maggor, "Politics of Property: Urban Democracy in the Age of Capital, Boston 1865-1900" (Ph.D. diss., Harvard University, 2010).

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