Coffin Planimeter Arm Signed by Ashcroft Manufacturing Company, No. 1423

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 vernier is corroded and may be a different type of metal from the rest of the instrument. The bar is marked: THE ASHCROFT MFG. CO. (/) — SOLE MANUFACTURERS. — (/) COFFIN'S PAT. JUNE 6. 1882. The underside of the bar is marked: No. 1423. The serial number suggests this object is not as old as MA*323705.
A wooden case is covered with dark brown leather and 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.
In 1912, company founder Edward H. Ashcroft sold his interest to Charles A. Moore, who renamed the firm Manning, Maxwell, and Moore (MM&M). Dresser Industries, Inc., purchased MM&M in 1964. Ashcroft survived as a brand name.
References: John Coffin, "Averageometer, or Instrument for Measuring the Average Breadth of Irregular Planes" (U.S. Patent 258,993 issued June 6, 1882); N. Hawkins, Hawkins' Indicator Catechism (New York: Theo. Audel & Co., 1903), 140–142; James Ambrose Moyer, Power Plant Testing (New York: McGraw-Hill, 1911), 73–78; David R. Green, "Coffin Planimeters," June 16, 2008,; Richard Oliver, "Ashcroft Manufacturing Co. History,"
Currently not on view
Object Name
date made
Ashcroft Mfg. Co.
Physical Description
german silver (overall material)
wood (overall material)
leather (overall material)
cloth (overall material)
case: 2.8 cm x 18 cm x 7.4 cm; 1 3/32 in x 7 3/32 in x 2 29/32 in
place made
United States: New York, New York
ID Number
catalog number
accession number
See more items in
Medicine and Science: Mathematics
Science & Mathematics
Data Source
National Museum of American History, Kenneth E. Behring Center


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