Felsenthal FAE-15 Stadia Computer Circular Slide Rule

This white plastic circular slide rule consists of a disc riveted to a square backing. The backing has a logarithmic scale of readings of a stadia rod used with a transit telescope, in feet. The disc has two logarithmic scales of angles. The first scale gives the difference in elevation of the transit and the stadia rod, in feet. It represents multiplying the stadia reading by 1/2 sin 2A, where A is the vertical angle of the transit telescope. The second scale finds the horizontal distance of the rod in feet and represents multiplying the stadia reading by the square of cos A. There is no indicator.

The instrument is marked on the front: STADIA COMPUTER. The interior of the disc has DIRECTIONS FOR USE and a table providing the quantity to be added when a constant is used in measuring stadia. On the back, the rule is marked: 6675-664-4676 (/) CONTRACT NO. DSA 700-68-M-AF86 (/) FELSENTHAL INSTRUMENTS CO. (/) CHICAGO, ILLINOIS (/) 22040 (/) MFR'S PART NO. FAE-15. It has a blue plastic case with snaps and a holder for a label. This object was donated with a second, duplicate Felsenthal stadia computer, which was assigned the same catalog number.

The instrument resembles Cox's Stadia Computer (see 1987.0221.01 and 1987.0221.02). Donor Ben Rau dated the object to 1968, which is consistent with the form of the company name on the instrument. For Felsenthal company history, see 1977.1141.01 and 1977.1141.02.

References: Deborah J. Warner, “Browse by Maker: Felsenthal,” National Museum of American History Physical Sciences Collection: Navigation , http://americanhistory.si.edu/collections/navigation/maker.cfm?makerid=173; accession file.

Currently not on view