Our museum is temporarily closed to support the effort to contain the spread of COVID-19. Read a message from our director, and check our website and social media for updates.

Felsenthal Range Deflection Protractor

Felsenthal Range Deflection Protractor

<< >>
Usage conditions apply
This clear plastic sixth-circle protractor is graduated into 10 groups of 20 divisions, each labeled with the number 50. The unit of angular measure is mils, which was utilized in artillery applications. There are 6,400 mils in a circle (i.e., 1 mil = 0.05625 degrees). The arc on this instrument is 1,000 mils, or 56.25 degrees, in length.
An arm extends from the left side of the protractor. The arm is divided to half-units and marked by five (scaled) meters from 160 to 10. Two holes with diameters of 5mm are next to the 145–140 and 30–25 marks. The vertex of the protractor bears a small notch.
The interior of the protractor is marked: PROTRACTOR, RANGE DEFLECTION (/) SCALE 1:25,000 (/) RANGE 16,000 (/) METERS. Stamped inside a box is the text: THIS TRAINING AID WILL BE RETURNED (/) TO THE FIELD ARTILLERY SCHOOL (/) UPON COMPLETION OF THE SUB-COURSE (/) WITH WHICH ISSUED. Donor Ben Rau dated the instrument to 1965.
See also 1977.1141.01, 1977.1141.02, 1977.1141.05, 1977.1141.08, 1977.1141.09, 1977.1141.10, 1977.1141.11, 1977.1141.12, 1977.1141.18, 1977.1141.19, 1977.1141.20, 1977.1141.21, 1977.1141.22, 1977.1141.23, 1977.1141.24, 1977.1141.30, and 1977.1141.39.
Currently not on view
Object Name
date made
Felsenthal Instrument Co.
Physical Description
plastic (overall material)
overall:.2 cm x 41 cm x 66.5 cm; 3/32 in x 16 5/32 in x 26 3/16 in
ID Number
accession number
catalog number
Credit Line
Gift of Ben Wharton Rau and Margery Felsenthal Rau
See more items in
Medicine and Science: Mathematics
Science & Mathematics
Data Source
National Museum of American History
Nominate this object for photography.   

Our collection database is a work in progress. We may update this record based on further research and review. Learn more about our approach to sharing our collection online.

If you would like to know how you can use content on this page, see the Smithsonian's Terms of Use. If you need to request an image for publication or other use, please visit Rights and Reproductions.


How is the protractor used?Thanks
The right side of the RDP rests on a pin on a 1:25,000 scale blank gridded chart. The pin represents the firing battery location. A second pin is inserted at the target location. The range to target is determined along the left side of the RDP. A third pin along the arc of the RDP determines the azimuth or direction of fire to the target. Quadrant elevation for the howitzers are determined using a specialized computer, or Tabular Firing Table (book), or Graphical Firing Table (slide rule). An observer will send subsequent corrections to the battery by radio or land line. Detailed explanation in US Army pub at link: http://www.bits.de/NRANEU/others/amd-us-archive/FM6-40%2884%29.pdf

Add a comment about this object