Subtense Instrument

an instrument used for the measurement and construction of angles. A subtense instrument is used in artillery when firing data are fully prepared by graphical methods and in topographic surveying with the aid of a telestereoscope. The instrument relates an angle to a chord on a circle with a given radius and is used instead of chord tables. The chord scale on a subtense instrument is not uniform. The accuracy of measurement with a subtense instrument is one graduation of the deflection scale, that is, 1/1,000 of the radius of the circle. Angles may be constructed or measured with greater accuracy by using a subtense instrument and a divider than by using a protractor or a celluloid protractor.