Patent GB-289995-A (GB)
Title: Improvements in or relating to alternating-current bridge measuring apparatus
IPC: G01R17/00
Filed: 1927-03-03
Abstract: 289,995. Place, W. I., and Metropolitan-Vickers Electrical Co., Ltd. March 3, 1927. No Patent granted (Sealing fee not paid). Inductances and capacities, measuring; magnetic tests and measurements. - Alternating- current bridges for measuring inductances and capacities are provided with an indicating instrument in which the moving coil is situated in an alternating magnetic field of the same frequency as that feeding the network. Fig. 1 shows a Wheatstone bridge having ratio arms comprising non-inductive resistances 10, 11 and a third arm containing a variable inductance 13 in series with a fixed resistance 14 and a variable resistance 15. The fourth arm contains the inductance 16 to be measured, which may surround a furnace so as to enclose a charge or a specimen within the furnace, in series with a resistance 17. The bridge network is energized from a three-phase star-connected sine-wave generator 18. The junction 19 of the ratio arms 10, 11 is connected through a variable inductance 20 to the end 21 of one phase winding of the generator 18, the junction 22 being connected to the end 23 of another phase winding. Across the points 24, 25 of the bridge is connected the moving coil 101 in series with a variable inductance 26. The coil 101 is mounted in the field of a coil 102 which is connected in series with a resistance 27 and an adjustable inductance 28 between the star point 29 and the end 30 of the third phase winding of the generator 18. The phase of the current through the field coils 102 is adjusted with respect to the bridge current by means of the variable inductances 20, 28 so as to make the instrument insensitive to unbalance of resistance while sensitive to unbalance of reactance. The inductance 13 may be replaced by the secondary coil of a mutual inductance, of which the primary coil is included in the connection between the points 22, 23. In order to detect the magnetic change point of a specimen, the coil 16 may surround the furnace, a cooling jacket being internosed between the coil and the furnace or the coil being of tubular conductor traversed by cooling fluid. The resistances 14, 17 may also be arranged within or adjacent to the cooling jacket so that they take up the same temperature as the coil 16. The bridge is balanced before introducing the specimen into the furnace. The specimen is then introduced, the balance being thereby disturbed. As the temperature rises the change point is approached, the exact point being indicated by the bridge again becoming balanced owing to the permeability of the specimen becoming united. Fig. 3 shows an induction balance in which the inductance 16 to be measured is connected in series with a variable inductance 13 and a resistance 15, with a variable tapping point 15<a>, between the ends 21, 23 of a star-connected three-phase generator 18. In the series circuit are included a variable inductance 50 and a variable resistance 51 for regulating the phase of the current through the inductance 16. Differentially acting coils 101<a>, 101<b> are connected across the inductance 16 and the inductance 13 and part of the resistance 15, and are arranged within the field of coils 102, which are connected across the third phase of the generator. The phase of the current through the field coils 102 is adjusted by means of the inductances 28, 50 and resistances 27, 51 so that the instrument is sensitive to resistance differences. The tapping point 15<a> is adjusted until balance is obtained. The phase of the field-coil current is then adjusted so that the instrument is sensitive to inductance unbalance. The variable inductance 13 is then adjusted until balance is again obtained, its value then being equal to that of the inductance 16. A change-over switch may be provided whereby the field coils may be connected across the points 21, 23 instead of the points 29, 30. The variable tapping point 15<a> may be omitted and a variable resistance arranged in series with the inductance 16. Fig. 5 shows a modification in which a mutual inductance is compared with an inductance. Alternating current is passed through a series circuit including a resistance 52, the inductance 16, and the primary winding 13<a> of the mutual inductance. The single moving coil 100 of the detector is connected across the inductance 16 and part or all of the resistance 52, in series with the secondary winding 13<b> of the mutual inductance, which is connected in opposition. Another form of induction balance in which a variable inductance is compared with a mutual inductance is shown in Fig. 6. Alternating current is passed through the variable inductance 54, a resistance potential divided 55, and the primary winding 56a of the mutual inductance. A series circuit is arranged from the point 57 through variable resistances divider 55, and the primary winding 56a of the mutual inductance to the adjustable point 60 of the potential divider. The moving coil 101 is connected between the junction of the resistances 58, 59 and junction of the potential divider 60 and inductance 50. The instrument is balanced for resistance bv adjusting the phase of the fieldcoil current and then balancing by means of the resistances 58, 59. The phase of the field-coil current is then readjusted and the variable inductance 54 adjusted to obtain balance. Reference has been directed by the Comptroller to Specifications 24442/11, 25044/11 and 219,640.