Circular Slide Rule, Bearing Rate Computer, Felsenthal FNR-5


Mathematical instruments have long been used to assist in aiming weapons. This circular slide rule is one such device. Made for use in antisubmarine warfare, it has two scales on the base, two scales on a concentric disc atop this, and a rotating pointer pivoted at the center. The lower disc has scales for the target speed in knots and the range in yards. The upper disc has scales for the bearing rate and angle on bow. The inner scales on both the base and the disc are in green.

A mark on the lower edge of the base reads: SPECIAL DEVICES CENTER DEVICE 1-BA-2a (/) BEARING RATE COMPUTOR [sic]. A mark on the disc reads:G. FELSENTHAL & SONS, INC. (/) PART NO. FNR-5.

Felsenthal made bearing rate computers for the U.S. Navy from at least the 1950s until at least the year 2000. A tag received with the object indicates that this model was produced from 1952.

Compare 1977.1141.33, 1977.1141.34, 1977.1141.35, and 1977.1141.36. Objects 1977.1141.35 and 1977.1141.36 appear to differ only in device number, the color of the two inner scales, and the spelling of “COMPUTER.”


U.S. Navy, Operations Specialist Volume I, NAVEDTRA 14308, 2000, p. 10-22 through 10-28. This volume was accessed online August 3, 2017.

Maker: Felsenthal Instrument Co.

Location: Currently not on view

Subject: Mathematics


See more items in: Medicine and Science: Mathematics, Military, Science & Mathematics, Slide Rules


Exhibition Location:

Credit Line: Gift of Ben Wharton Rau and Margery Felsenthal Rau

Data Source: National Museum of American History

Id Number: 1977.1141.34Catalog Number: 336418Accession Number: 1977.1141

Object Name: slide rule

Physical Description: plastic (overall material)metal (overall material)Measurements: overall: .2 cm x 15.2 cm x 15 cm; 3/32 in x 5 31/32 in x 5 29/32 in


Record Id: nmah_1213911

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