Patent GB-194780-A (GB)
Title: An apparatus for effecting electrical measurements and tests on the principle of the wheatstone bridge
IPC: G01R31/08
Filed: 1921-12-15
Abstract: 194,780. Planer, V. Dec. 15, 1921. Resistances, measuring; combinations of instruments; variable resistances. - Belates to wheatstone bridges for comparing electrical quantities in terms of resistance, or for locating faults in electric cables or insulated lines by connecting the beginning of the cable to the end of the bridgeresistance wire, the end of the cable to a movable contact on the wire, a galvanometer or other zerocurrent indicator across the ends of the cable, and a current course, earthed at its other pole, to a second movable contact on the wire, the indicator and source being interchangeable. The resistance wire is arranged in spiral windings and the two movable contacts are independently moved along the wire by means of crank drives actuating recording mechanisms. In the localization of faults, the first drive is adjusted until the corresponding recording mechanism indicates the known length of the cable, and the second until the zero-current indicator gives a normal reading, when the distance of the fault from the beginning of the cable is read on the second recording mechanism. In measuring resistances, the unknown resistance connected in series with a known resistance replaces the cable and is measured bv observing the ratio of the readings on the recording mechanism when the drives are adjusted to give zero current in the radiator. In the form shown in Fig. 3, the resistance wire 2 is wound spirally on an insulating drum 1 rotated by a crank drive 14. The beginning of the cable is connected to the end of the wire 2 through a conducting ring 3 on the drum 1 and a brush 4. When the crank 14 is rotated. the first movable contact 5 connected to the end of the cable is moved axially along the drum by means of a longitudinal guide 8 and a screw 6 rotated by gearing from the drives 14 in such a manner that the brush 5 travels along the length of and constantly contacts with the wire 2 The second movable contact 20 connected to the current source or current indicator is adjusted by rotation of a crank 22 connected to an arm 21 carrying a guide 16. The guide 16 passes through an aperture in a dise 17 which carries the holder 19 for the contact brush 20 and which engages a fixed screw spindle 15, the dise 17 being screwed along the spindle 15 so that the brush 20 describes a spiral path along the length of the wire 2 and the drun 1 and wire 2 being stationary during this adjustment. The end of the serew 15 may be mounted in the drum 1. Pinions 12, 18 actuate recording mechanisms. The contacts way comprise shallow cups or resilient rings around the drum 1. In a modification, the resistance wire is wound in a spiral groove in a stationary drum and both movable contacts are actuated in the same manner and by the same mechanism as the contact 20, Fig. 3. The ends of the two fixed screw spindles of the contact-actuating mechanism may then support the drum, and the contacts may comprise wheels running along the wire and rotatablv mounted in their holders. In a further modification, Fig. 5, the movable contacts 5, 20 are mounted at the ends of arms 84, 66 which extend through longitudinal slots 88 in sleeves 79, 63 and carrv screw bosses 83, 65 within the sleeves and engaging fixed screwed spindles 78, 64. The resistance wire 2 is wound under tension in a spiral groove in a fixed insulating drum 1 carried bv arms on a boss 77 mounted on the spindles 78, 64, the outer ends of the spindles being mounted in the housing 58. The sleeves 79, 63 are rotatably mounted on turned bearings on the spindles 78, 64 and, in order to move the contacts 5, 20 spirally along the wire 2, the sleeves are rotated by external crank drives through teeth 80, 62 engaging worm wheels 81, 61. The contact 5 is connected to an external terminal for the end of the cable bv a resilient member 85 insulated from the arm 84 and having contact over a large area with a fixed conducting ring 87 coaxial with the drum 1. The contact 20 is connected to the current source through a line 73 mounted on the arm 66 and leading to a collector 69 rotating with the arm 66 and contacting with a fixed collector 68. The collector 69, Fig. 10, may comprise a spring ring connected to the line 73 by two branches 89, and the collector 68 a collector ring mounted on a fixed insulating sleeve 153. The lines 67, 90 leading respectively to the collector 68 and to the end of the wire 2 mav pass through an axial aperture in one of the spindles 78, 64. The contacts 5, 20 may comprise hooks surrounding the wire 2 and lifting it out of the groove in the drum 1. The current source is connected to a second external terminal for earthing and, through a push-button or other switch, to the line 67 and current collector 68. The zero-current indicator is connected through a push-button or other switch to a third external terminal for the beginning of the cable to which the end of the wire 2 is also connected through line 90, the other terminal of the indicator being connected to the ring 87. The housing 58 encloses the wire 2 and contacts 5, 20, the recording mechanisms and the current source and indicator, carries devices for operating the switches, the cranks and the three external terminals one for earthing and two for the cable, and is provided with inspection apertures for the recording mechanisms and for the indicator. The current source and indicator are located in parts of the housing easily accessible by raising covers. The three terminals may be mounted on a detachable cover.