Patent GB-598243-A (GB)
Title: Improvements in and relating to the location of conducting and/or magnetic bodies
IPC: G01V3/10
Filed: 1945-01-24
Abstract: 598,243. Locating conductors; magnetic tests and measurements. BRITISH ELECTRICAL & ALLIED INDUSTRIES RESEARCH ASSOCIATION, WHITEHEAD, S., and ROSENBLUM, B. Jan. 24, 1945, No. 1959. [Class 37] Equipment for the location of conducting bodies, and magnetic bodies, comprising means for producing an alternating magnetic field over a relatively large area, and search equipment for searching the area and whose dimensions are small relatively to the area, comprises a plurality of search coils, means for supporting the coils in fixed relation to one another, and means for indicating out-of-balance electromotive forces in the coils, the coils being so arranged that when the search equipment is placed with the magnetic field linking the coils and with an axis of the search equipment in either of two mutually perpendicular positions in a plane perpendicular to the magnetic field and when the field is non-uniform and has a substantially linear gradient, the electromotive forces in thecoils substantially balance one another. The area to be searched lies within or without a loop of cable fed from a 500 cycle A.C. supply. Four search coils 13-16, Fig. 2, wound in a similar manner are rigidly fixed upon a support with their axes parallel. Search coils 13, 16 are connected across terminals 23, 25 of a resistance 24, and search coils 14, 15 are connected across terminals 23, 26 of this resistance so that, when the coils are traversed by like magnetic fields, the potential difference between terminals 25, 26 is zero. The presence of conducting or magnetic bodies causes unbalance in the coils and consequently an indication by a telephone 32 connected across the resistance 24 through an amplifier 31 and potentiometers 52, 53. Exact voltage balance between the two pairs of coils 13, 16 and 14, 15 in the first instance, when the unit is moved in horizontal and vertical planes, is obtained by adjustment of the tappings 23, 39, by slight adjustment of one of the coils 13, 16, or by trimming coils 27 ... 30 connected in series with and disposed co-axially within coils 13 ... 16. Balance as regards forward or lateral rotation of the unit about horizontal axes is secured by means of coils 18, 19 which are mounted on the same support as the coils 13 ... 16 with their axes at right-angles to each other and to those of the search coils ; the voltages from these coils are applied between terminals 25, 41 and terminals 26, 42 respectively, any phase difference between these voltages and that across the resistance 24 being reduced by changing the value of the resistance 36. In order to reduce the effects of variations in the mean magnetic field the potential difference between points 23, 40, which is approximately proportional to the mean magnetic field and is derived from a balancing circuit comprising resistors 24, 37, condensers 34, 35, which are equal in value, and a condenser 38 are applied to the input of the compensator, the D.C. output of which is fed partly to the control grids of the amplifier and partly to the control grid of the compensator. The ratio of these voltages is so arranged that the output of the amplifier is made independent of the mean field strength. Compensation for space differentials of second and higher orders may be balanced by the use of a multiple of four coils. Specification 598,244 is referred to in the Provisional Specification.