Patent GB-801924-A (GB)
Title: Improvements in or relating to electrical measuring apparatus for current or magnetic fields
IPC: G01R33/04
Filed: 1955-10-11
Abstract: 801,924. Voltage ; magnetic measurements. COMPAGNIE POUR LA FABRICATION DES COMPTEURS ET MATERIEL D'USINES A GAZ. Oct. 11, 1955 [Oct. 22, 1954], No. 28915/55. Class 37. An electric measuring apparatus comprises a moving coil galvanometer whose coil is without restoring torque to which is applied a current to be measured, a control device for the output of an oscillator varied by the coil deflection, and an impedance energized by the oscillator output current whose voltage drop energizes an amplifier detector exciting a constant speed separately excited generator, whose armature energizes a load circuit and a compensating circuit which opposes the moving coil current proportionally to the armature output voltage so as to zeroize the coil current and make the current in the load and compensating circuits a measure of the unknown current; the galvanometer coil being, for example, energized by a magnetically sensitive device producing a current representing an ambient magnetic field and the compensating circuit comprising a solenoid energized from the armature to produce a field in neutralizing opposition to ambient. An unknown unidirectional voltage is applied to input terminals 1, 2 (Fig. 1) and the resultant current in resistance 13 and galvanometer moving coil 3 without restoring torque deflects the coil, which is coupled to the moving plate of variable capacitance 10 coupling H.F. oscillator 8 to impedance 11 at the input of detector amplifier 12, which energizes the excitation winding 17 of motor-generator 15, whose output current indicated by instrument 14 opposes the current in resistance 13 due to the input voltage, so that at balance the currents are equal and zero current flows in the moving coil; the current shown by instrument 14 being a measure of the input voltage. In a modification, (Fig. 2, not shown), the detector oscillator output may excite an A.C. fed magnetic amplifier whose output is rectified to energize the field winding of the motor generator. Alternatively, the motor generator may be excited direct from the detector oscillator output, to energize the magnetic amplifier, whose rectified output is connected to oppose the current in resistance 13 due to the input voltage. In Fig. 3, two solenoids 33a, 33b and a third solenoid 39 are wound on high permeability ferromagnetic cores 34a, 34b; 33a, 33b being - excited in antiphase through non-linear resistances 35a, 35b from transformer 32, while 39 is energized from motor-generator 15 in series with compensating winding 40. Moving coil 3 without restoring torque is energized across resistance 38 joining the centre tap of the secondary of transformer 32 to the junction of windings 33a, 33b; and is coupled mechanically as above to vary capacitance 10 coupling oscillator 8 to the input of detector-amplifier 12 energizing the excitation winding 17, so that for absence of ambient magnetic field in solenoids 33a, 33b the moving coil 3 is undeflected and windings 17, 39 are de-energized. The presence of an ambient field produces asymmetric magnetization of the solenoids, and due to non-linear resistances 35a, 35b a unidirectional current flows in moving coil 3. Its deflection produces a unidirectional current dependent on the ambient field in winding 17, and the output of the motor generator 15 flowing in winding 39, ammeter 41, and winding 40 is varied until at balance it neutralizes the ambient field in cores 34a, 34b whose value is indicated by the ammeter 41. The field of coil 40 is such as to neutralize the ambient field therein.