Patent GB-528722-A (GB)
Abstract: 528,722. Current, voltage, and power measurements ; calibrating instruments ; compensating instruments for temperature errors. ELLIOTT BROS. (LONDON), Ltd., SHOTTER, G. F., and HAWKES, H. D. May 13, 1939, No. 14356. [Class 37] An ironless dynamometer instrument is combined with a D.C. instrument for supplying the control torque, means for connecting all the coils of both instruments in series, and means such as a magnetic shunt for adjusting the torque of the D.C. instrument to balance that of the dynamometer. A dynamometer 1 and a permanent magnet moving coil instrument 2 are mounted one above the other with their moving coils 3, 4 in the same plane and fixed to a rod 5 supported by phosphor'bronze suspensions 6. The rod 5 carries a mirror 8 and also a pointer moving over a scale. Each of the parts 1, 2 is enclosed in a screening box 1a, 2a of iron or other high permeability material. The permanent magnet 11 is provided with a magnetic shunt in the form of a disc 12 movable towards or away from the pole pieces by a screw 13 for zero adjustment. To compensate for any magnetic leakage through the screening a small bar magnet 32 is adjustable to neutralize stray field outside the screening. Temperature compensation is effected by a variable tapping on a resistance in series with the coil 3 which is moved in accordance with the voltage drop across tappings 35, 36, Fig. 3, on one of the fixed coils 27. The lower part 2 is mounted on the underside of a plate 14 supported on pillars 15 carried on a base 16. On the upper side of the plate 14 is a bracket 17 supporting the upper part 1 and its screening casing 1a. In the space between the base of the bracket 17 and the plate 14 is the mirror 8, whilst immediately above the base are two further magnetic screens 18. The whole assembly is enclosed in a wooden case 19 through which project a knob 20 for adjusting the magnetic shunt 12 and the control 21 for. the suspension 6. A window 24 allows a beam of light from a projector 25 to pass on to the mirror 8. The instrument is standardized by connecting the moving coils 3, 4 oppositely in series to a D.C. source 26, Fig. 3, the fixed coils 27 also being connected in series with the moving coils. The current is measured by a potentiometer and standard cell connected across a standard resistance 28. The magnetic shunt 12 is then adjusted until the coils 3, 4 take up a zero position. For measuring power in an A.C. circuit 29, Fig. 24, the moving coil 3 of the, dynamometer is connected in series with a resistance 30 across the circuit 29, the fixed coils 27 being connected in parallel, as shown, or in series across the secondary of a standard current transformer 31, the primary of which is connected in series with the load. The coil 4 is excited by direct current from the source 26, its direction being such as to produce a torque opposing that of the coil 3. The value of the D.C. is adjusted until the torques balance, giving zero deflection, and is measured by a potentiometer across the standard resistance 28, or by a standard ammeter. The connections of the dynamometer may be altered for measuring alternating current or voltage. For standardizing an A.C. ammeter, voltmeter, or wattmeter, the instrument is first connected as shown in Fig. 3 and standardized with a current, e.g. 50 milliamperes, corresponding to the full scale deflection of the instrument to be tested. The A.C. circuit is then adjusted to cause full scale deflection of the instrument under test and the dynamometer 1 is connected to the A.C. circuit in the appropriate manner. The D.C. circuit is then adjusted until 50 milliamperes flows through the moving coil 4, as measured by the potentiometer across the resistance 28, and the A.C. circuit is adjusted until the moving coil system 3, 4 is brought to zero. The error in the deflection of the instrument under test is then read off. The potentiometer may be calibrated to read directly the current, voltage, or power in the A.C. circuit. The permanent magnet 11 may be replaced by an electromagnet excited from a separate D.C. source.