Patent GB-530698-A (GB)
Abstract: 530,698. Current, resistance, phase-difference, frequency, and capacitance measurements. NALDER BROS. & THOMPSON, Ltd., and LIPMAN, C. L. June 14, 1939, No. 17387. [Class 37] A measuring instrument or relay has at least two moving coils which are fixed to a common spindle not passing through either coil and embrace a magnetic pole piece or pole pieces so shaped as to allow the coils to move bodily. The cross-sectional area of the pole piece varies so that the flux distribution along the path of each coil varies, the gap being of constant depth. The axial thickness of the pole pieces is constant but the radial width varies. A permanent magnet 1, magnetized. in the direction of the arrow 2 has a ring-shaped soft iron pole piece 3, which may be solid or laminated. The pole piece 3 is embraced by coils 4, 5 fixed to a spindle 6. The pole piece is in halves to enable it to be threaded through the coils and is fixed by screws 7 to an arm 1a by which it is connected magnetically to the lower pole of the magnet, a distance piece 8 being interposed between the pole piece and the opposite pole of the magnet. The pole piece 3 is of constant axial depth but its radial width varies from a minimum at 9 to a maximum at 10, so that as one of the coils 4, 5 moves into a stronger field the other moves into a weaker field until a position of equilibrium is taken up corresponding to the ratio of the currents in the coils. In a modification, one pole may be bifurcated so as to form two pole pieces lying on opposite sides of the pole piece of opposite polarity, the two coils embracing the same or different pole pieces. The invention is applicable to ammeters, voltmeters, ohm-meters, ratiometers, phase meters, capacitance meters &c. In an ammeter or voltmeter one of the moving coils may carry a constant current, thereby taking the place of a spring. The other coil is the usual measuring moving coil. Alternatively, the force provided by one coil may be used to modify or supplement the action of the usual spring. In a single-phase power-factor meter the field is provided by an electromagnet energized from a current transformer, which is itself energized from the A.C system the power factor of which is to be measured. The two moving coils are connected in two parallel circuits across the secondary of a voltage transformer energized from the system, one coil being in series with a non-inductive resistance while the other is connected in a reactive circuit. When applied to a frequency meter and the field is provided by a permanent magnet the two moving coils are connected across rectifiers which are connected in parallel circuits across the A.C. system the frequency of which is to be measured. The parts of the parallel circuits between the rectifiers and the A.C. system have impedances of different character, one being resistive and the other reactive. The latter circuit or both circuits may be tuned in order to open the scale. When the field is provided by an electromagnet, the winding of the magnet forms a third parallel circuit across the A.C. system, the moving coils being connected directly in their parallel circuits without rectifiers. Specifications 456,988 and 469,938 are referred to.