Patent GB-753495-A (GB)
Title: Improvements relating to electrical circuits
IPC: H03H2/00
Filed: 1953-08-05
Abstract: 753,495. Determining physical properties electrically. VARIAN ASSOCIATES. Aug. 5, 1953 [Aug. 8, 1952], No. 21598/53. Class 37. The invention relates to apparatus for producing zero electric coupling between two coils which are placed very close together. The coils are associated with a generally cylindrical plastic body 1 (Fig. 1) which has two longitudinal central holes 3, 4, each containing a plastic, rotatable plug 7, 11 respectively. Plug 7 has a half disc cut away at the inner face, into which is fitted a copper sheet 9; plug 11 has embedded therein a resistor 14 (Fig. 4), short-circuited through a conductor which includes one or more turns of a coil 15, and a diametric portion 16 on the inner face. Between the two holes 3, 4 is a perpendicular hole 6, wherein material to be tested is located in a plastic capsule 17 inside a receiver coil 20, and a transmitter coil 30 is located on a plastic tube 2 covering the cylindrical body 1, two further coils 31, 32 (Fig. 3), being spirally wound on the curved outer surface of the tube 2, with a common axis which is mutually perpendicular to the axes of coils 20, 30. An electrostatic shield is located between the sweep coils 31, 32 and the other coils 20, 30 comprising interlaced copper wires 37, 38, connected to conducting rings 34, 35, respectively. The stray coupling, direct and indirect, between coils 20 and 30 produces the normal voltage component E1 (Fig. 6), in receiver coil 20, 180 degrees out of phase with the transmitter voltage, and a component E2 lagging by 90 degrees, produced by eddy current flow, &c. Considering the copper sheet 9 in plug 7 as primarily reactive, it produces in coil 20 a voltage 180 degrees out of phase with E1, and of variable magnitude obtained by rotation of a shaft 8. The current produced in the resistor loops 14, 15, 16 lags behind E by a small angle and induces a voltage E3 or E7 in coil 20 whose magnitude and sign depends on the position of the loop 16 relative to the coil. Thus, to obtain zero coupling between coils 20 and 30, plug 11 is rotated to a position where the fixed, stray voltage component E2 is cancelled by component E4 from the resistor loop 14, 15, 16, and plug 7 is rotated to a position where the sum of the voltage components E1 obtained by direct coupling between coils 20 and 30, and E5 obtained by coupling between coil 20 and the resistor loop 14, 15, 16 cancelled by the voltage induced by the copper sheet 9. The resistor loop 14, 15, 16 may alternatively be an oblique loop of resistance wire, or a disc of resistive material, e.g. carbon, secured to the end surface of the plug. The apparatus is described as being used in the chemical analysis of material by nuclear magnetic resonance, wherein the angular rate of precession # o of the nucleus of the material, in the presence of a constant magnetic field H o , is given by ÁHo/I, where Á is the magnetic moment of the nucleus, and I is its gyroscopic moment. If a further magnetic field H 1 , rotating at an angular velocity #, perpendicular to field H o  is added, the precession is at a small angle to the field H o  except when # is equal to #o, when the angle is 90 degrees. Thus a coil at 90 degrees to both H o  and H 1  will have a voltage induced in it only when # equals # o , which voltage can estabilish the identity of the material whose nuclei is being examined. In use, the test material is located in the capsule 17, the apparatus is located between the poles 44 (Fig. 5) of a magnet, sweep coils 31, 32 are fed with mains frequency A.C. to produce precessing at velocity # o , and transmitter coil 30 is fed with R.F. power at velocity # from a transmitter 48. When # is equal to # o a voltage is picked up by receiver coil 20, which after amplification at 49, 50, 51, is indicated by a cathode-ray oscilloscope 47. U.S.A. Specification 2,561,489 is referred to.