Monroe Model K-20 Calculating Machine


This example of Monroe Calculating Machine Company’s Model K-20 calculating machine was lent to the Smithsonian in 1922 as an example of one of the company’s latest products.

The manually operated, full-keyboard, modified stepped drum machine has a metal (not steel) frame painted black, with rounded corners. It slopes downward from back to front. The steel plate under the keyboard is painted green. The ten columnsof plastic keys are colored black or white according to the place value of the digit represented. A red clearance key is at the bottom of each column. Keystems are of uniform length. Rods between the rows of keys serve as decimal markers. They are painted green on one side to match the plate under the keyboard, and unpainted on the other side.

A column to the right of the number keys has three keys. One clears the entire keyboard. The other two are set to determine whether or not the keyboard clears after each calculation. A metal lever is to the right of the keyboard and a metal knob to the left. The operating crank on the right side rotates clockwise for addition and counterclockwise for subtraction.

In back of the keyboard is a carriage that has a row of 20 numeral dials for recording results, and a row of ten numeral dials behind these which serve as a revolution register. Two thin metal rods between the windows carry decimal markers. The carriage shift crank is at the front of the machine. A knob for lifting the carriage is to the right of the result register, and a crank for zeroing dials on the carriage is further to the right. This crank lifts the carriage as it resets the dials. The machine has four rubber feet.

A mark at the center front reads: MONROE (/) Calculating Machine Company (/) New York,U.S.A. A mark at the center of the back reads: MONROE. The serial number, 50885, stamped the on right side of the mechanism of the machine, under the carriage, is difficult to read.

Compare MA.334711, MA.307386, 1983.0831.01, and 1982.0682.05..


J. H. McCarthy, The American Digest of Business Machines, Chicago: American Exchange Service, 1924, p. 551.

Date Made: 1921

Maker: Monroe Calculating Machine Company

Location: Currently not on view

Place Made: United States: New Jersey, Orange

Subject: Mathematics


See more items in: Medicine and Science: Mathematics, Calculating Machines, Science & Mathematics


Exhibition Location:

Credit Line: Gift of Monroe Systems for Business

Data Source: National Museum of American History

Id Number: MA.307386Catalog Number: 307386Maker Number: 50185Accession Number: 2011.0209

Object Name: calculating machine

Physical Description: steel (overall material)wood (overall material)aluminum (overall material)plastic (overall material)rubber (overall material)Measurements: overall: 19.7 cm x 54 cm x 39.5 cm; 7 3/4 in x 21 1/4 in x 15 9/16 in


Record Id: nmah_690557

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