GB352254A - Improvements in or relating to field instruments for measuring angles - Google Patents

Abstract

352,254. Protractors. DAHLBERG, L. E. M. C., Kristianstad, Sweden. July 10, 1930, No. 20954. [Class 97 (iii).] An angle-measuring instrument comprising a transparent plate has a central hole preferably metal tipped, double angle-graduations extending for 360‹ in both directions from a zero-line, and means for accentuating the zero-line to enable marks along it to be made in any position of the instrument. As shown in Fig. 1 the instrument comprises a square celluloid plate 21 from the centre 22 of which run lines ending in the outer graduated circle 25. The slot 23, parallel to one side of the plate, rims from near the hole 22 to near the periphery, and has a distance scale 28. Lines 29 parallel to the slot 23 permit of adjustment along the meridian. Holes 30 at definite distances from the centre serve for fixing the plate, and the chequered portion 31, Fig. 5, for making sketches. Angles subtended by distant objects may be measured by holding the scale 34 graduated correspondingly to the scale 26 at a definite distance from the eye by means of a strap fastened to the hole 33. The scale 34 is completed by a toothed opening 37. A slot 24 is aligned with the slot 23. The instrument shown in Fig. 5 has a spiral scale 40, having one turn per unit of length and being subdivided into equal arcs by radii. Distances are measured on a map from the centre 22 to the appropriate point on the spiral. The slot 23 may be replaced by a stretched wire, the bevelled edge of an opening in the plate, or by a sharp edge formed on the underside of the plate. The plate may only comprise a portion of the whole circle.