Coffin Planimeter Arm Signed by Ashcroft Manufacturing Company, No. 830

This German silver instrument is a curved bar with a short arm, on which a short cylindrical weight is placed, and a longer arm, to which a tracer point is screwed. The middle of the bar has a thin cross-rod, on which a measuring wheel rotates against a wedge-shaped vernier. The wheel is numbered from 0 to 14, with each unit divided into five parts. The bar is marked: THE ASHCROFT MFG. CO. (/) — SOLE MANUFACTURERS. — (/) COFFIN'S PAT. JUNE 6. 1882. The underside of the bar is marked: No. 830. The serial number suggests this object predates 1987.0107.03. Compare also to MA.323708 and MA.323706.

A wooden case covered with dark brown leather is lined with purple silk and velvet. The top of the case is marked: MADE BY (/) THE ASHCROFT MANF'G CO. (/) NEW YORK & BRIDGEPORT.

John Coffin of Syracuse, N.Y., applied for a patent on this variation on a planimeter in July 1881. He designed his "averageometer" to calculate areas in diagrams of work performed by steam engines. The Ashcroft Manufacturing Company of New York City and Bridgeport, Conn., was the first of several American firms to make the device.

Ashcroft, best known for making pressure gauges for steam engines, often sold the arm for Coffin's planimeter separately from its base, as in this example. In 1910, Frederick C. Blanchard, Ernest B. Crocker, and Philip G. Darling, who all probably worked for Ashcroft, patented an improvement to Coffin's planimeter so that it could be clamped in place. The planimeters made by Ashcroft after 1910 utilized this improvement, so this example was made between 1882 and 1910.

The instrument was received at the Smithsonian in 1964.

References: John Coffin, "Averageometer, or Instrument for Measuring the Average Breadth of Irregular Planes" (U.S. Patent 258,993 issued June 6, 1882); F. C. Blanchard, E. B. Crocker, and P. G. Darling, "Planimeter" (U.S. Patent 961,836 issued June 21, 1910); David R. Green, "Coffin Planimeters," June 16, 2008, http://www.planimetervault.com/coffin.html; N. Hawkins, Hawkins' Indicator Catechism (New York: Theo. Audel & Co., 1903), 140–142.

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