GB741160A - Improvements relating to electrical measuring instruments - Google Patents

Abstract

741,160. Current measuring instruments. NATIONAL RESEARCH DEVELOPMENT CORPORATION. June 8, 1954 [May 6, 1953], No. 12632/53. Class 37. [Also in Group XXXVIII] A sensitive critically damped low inertia galvanometer for indicating the strength and duration of short current pulses comprises plural lengths of fine wire hanging in parallel slack catenaries between insulating supports in a narrow catenary shaped airgap between correspondingly shaped polepieces establishing therein a magnetic field in a vertical plane at right-angles to each catenary at all points of its length, and the wires are connected respectively in series with plural thermojunctions disposed along the anticipated line of fracture of a test piece to detect the temperature wave set up by energy released at the instant of fracture; the deflections of the wires being displayed or recorded by optical projection. Soft iron yokes 1, 6 (Fig. 1) having adjacent surfaces 2, 7 of catenary shape are clamped at their ends on opposite faces of bar magnets 4 and a front cover-plate 14 (Fig. 2) is fitted with pivoted arms 15 located by stops 17 adjustable in arcuate slots 18 to carry insulating pegs 10 and terminal blocks 9 to which are attached plural fine copper wires 8 of 0.008 inches diameter whose slackness is adjustable by positioning arms 15 so that they hang freely in the catenary airgap between yokes 1, 6, which is traversed by a magnetic field of 500 lines per square centimetre in a vertical plane normal to the wires at all points of their length. An aperture 11 in yoke 1 accommodates a projection lamp (not shown) and the images of the catenary wires are focused by a lens system 13 on a calibrated scale or photographic plate. A mask having a slit perpendicular to the wires may be interposed between the lens and the plate, and uniformly moved along the line of the wires as a time axis by the distortion of a bimetallic strip which is electrically heated from the switchable circuit of the projection lamp. Each catenary wire is connected in series with its associated thermojunction disposed along the anticipated fracture line and with another cold thermocouple at a remote point of the test piece so that the temperature wave at each thermocouple may be measured or recorded as the fracture progresses.