A Galvanometer for Alternate Current Circuits

Abstract

Tests of effects due to change of current, such as induction phenomena, are often hard to carry out because the ballistic galvanometer available is not sensitive enough. Alternate current tests are still less satisfactory owing to special difficulties. The vibration galvanometer overcomes only a few of these difficulties. It must be adjusted to resonance for the best effects, and its indications vary with current frequency. The sensitiveness of ballistic galvanometer tests can be greatly increased by the use of some form of mechanical commutator by means of which a crude form of alternate current is produced. A better method would be to generate the current in the usual way if a suitable instrument existed. The galvanometer here described is the result of an attempt to construct a measuring instrument by means of which inductances and capacities can be compared by bridge methods as accurately as it is possible to compare resistances. The instrument is like a moving coil galvanometer in almost every respect, except that its field is due to a specially constructed electromagnet excited by an alternating voltage. This voltage V is applied to a winding of m turns of the electromagnet, and the core flux N is such that V = rA + mN, where r is the resistance of the winding and A the current traversing it. The coil and electromagnet are so designed that for currents of the frequencies used the value of rA is negligible in comparison with V. The rate of change of N will therefore be at each instant a measure of V, whatever the permeability or hysteresis of the core. The instrument has a laminated electromagnet formed of stampings of two kinds - a rectangular portion with two straight limbs forming the core of the electromagnet, and a specially shaped stamping between the poles. The moving coil of 50 turns swings in a narrow gap separating the stampings, in much the same way as in a permanent magnet instrument. On the limbs of the magnet are windings of 200, 2000, and 4000 turns. The iron will not be too strongly magnetised if the winding used contains 20 turns per volt on 50 similar circuits, but the instrument is so sensitive that such excitation will only be needed for exceptional tests. If a voltage V be applied to one of the field coils of m turns, and if the same, or another, field winding of n turns be joined up, through a condenser of K microfarads, to the moving coil, the torque acting on this moving coil will be a measure of Kn(V/m)2; i.e. the deflexion is proportional to the square of the voltage. By suitably choosing K, m and n, the voltmeter may be used over a large number of ranges. Thus with the instrument shown a deflexion of 200 mm. on a scale at 1 metre distance, can be obtained either for 200 volts or for 20 millivolts. The deflexion is independent of frequenev and wave-form if the field-winding to which the voltage is applied has a resistance negligible in comparison with its impedance. Thus with m=4000 and n=200 it was found that the value of V2 required to give a certain deflexion was independent of frequency between 50 similar and 100 similar, but at 25 similar it was 6½ per cent. less than at 50 similar. At any fixed frequency the deflexion will always measure KV2. The instrument may be used with great advantage to compare inductances and capacities by the ordinary bridge methods, the working conditions being (i.) the alternating voltage V applied to the field coil of the instrument must also cause the current in the bridge conductors, (ii.) the alternate current in the bridge must be made in phase with the voltage V by the use of suitable non-inductive resistances, (iii.) the moving coil must be placed directly across the bridge. The balance can be adjusted with ease to 1 part in 10,000 when the voltages set up on the coils or condensers are of the order of 1 volt. When a balance of great precision is needed, the minute electromotive force e induced in the moving coil by the alternating field of the magnet, tends to cause a small deflexion disturbing the balance. When the moving coil circuit is non-inductive, the current due to e will be in phase with e and in quadrature with the flux so that the corresponding deflexion will be negligible. But in all cases any effect due to e can be accurately eliminated by working to a false zero. As illustrations of the behaviour of the instrument, the results of tests are given on the measurement of the mutual induction of coils, the comparison of capacities, and the measurement of Specific Inductive capacity.